S ecuring the international borders of the United States has been an issue of perennial interest and importance to the federal government. Federal law authorizes the Department of Homeland Security (DHS) to construct barriers along the U.S. borders to deter illegal crossings. DHS is also required to construct reinforced fencing along at least 700 miles of the land border with Mexico, though fencing is not required to be deployed at any "particular location" alon g that border. Responsibility for carrying out these functions, and more generally securing the U.S. land borders between ports of entry, primarily falls to U.S. Border Patrol within DHS's Customs and Border Protection (CBP). Although congressional interest in the legal framework governing fence deployment has, in recent years, tended to focus on the stringency of the statutory mandate to deploy fencing along at least 700 miles of the southern border, attention has now shifted to those provisions in current law which allow, but do not require, the deployment of fencing and other barriers along additional portions of the U.S. land borders. Within days of taking office, President Donald J. Trump issued an executive order instructing the Secretary of Homeland Security, acting under existing legal authorities, to "take all appropriate steps to immediately plan, design, and construct a physical wall along the southern border ... to most effectively achieve complete operational control" of the U.S.-Mexico border. The order defines a "wall" to mean "a contiguous, physical wall or other similarly secure, contiguous, and impassable physical barrier." The order does not identify the contemplated mileage of the wall to be constructed. The primary statute authorizing DHS to deploy barriers along the international borders is Section 102 of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA). Congress made significant amendments to IIRIRA Section 102 through three enactments—the REAL ID Act of 2005, the Secure Fence Act of 2006, and the Consolidated Appropriations Act, 2008. These amendments established a mandate upon DHS to construct hundreds of miles of new fencing along the U.S.-Mexico border, and also provided the Secretary of Homeland Security with broad authority to waive "all legal requirements" that may impede construction of barriers and roads under IIRIRA Section 102. These statutory modifications, along with increased funding for border projects, resulted in the deployment of several hundred miles of fencing and other barriers along the southwest land border between 2005 and 2011. A portion of this infrastructure is fencing that is primarily intended to prevent illegal border crossings by foot (referred to by DHS as "pedestrian fencing"). Other types of barriers have been installed to impede vehicles from smuggling persons or contraband into the United States (referred to by DHS as "vehicle fencing" ), but do not stop crossings by persons traveling on foot. In some instances, an additional layer of fencing (secondary fencing) may also be installed behind primary pedestrian fencing to further impede illegal crossings. The efficacy of deploying additional fencing along the border has been the subject of debate among many policymakers, particularly regarding the cost, environmental impact, and effectiveness of border fencing in comparison to alternative means of deterring illegal crossings. Largely on account of changes in DHS's border enforcement strategy and prioritization of resources during the Obama Administration, the construction of additional fencing along the land border with Mexico has largely halted. In October 2014, DHS indicated that it had constructed a total of 352.7 miles of pedestrian fence (in addition 36.3 miles of secondary fencing), and 299 miles of vehicle fencing along the southwest border. The total amount of pedestrian and vehicle fencing identified by DHS was slightly less than the 653 miles that U.S. Border Patrol had reportedly identified as appropriate for fencing and other barriers. However, it appears that further fencing would need to be deployed in order for DHS to satisfy the statutory requirement that the agency construct fencing "along not less than 700 miles of the southwest border." Proposals were introduced or considered by Congress in recent years to modify statutory authorities governing DHS's deployment of fencing and other infrastructure along or near the border, including by calling for the deployment of additional fencing, but such legislation has not been enacted. DHS's policy not to deploy a substantial amount of additional fencing, beyond what is expressly required by law, appeared to be premised primarily on policy considerations and funding constraints, rather than significant legal impediments. Indeed, nothing in current statute would appear to bar DHS from installing hundreds of miles of additional physical barriers along the U.S.-Mexico border, even beyond the 700 miles required by law, so long as the action was determined appropriate to deter illegal crossings in areas of high illegal entry or was deemed warranted to achieve "operational control" of the southern border. This report provides an overview of the key statutory authorities and requirements governing DHS's construction of barriers along the international borders of the United States. It also includes appendixes listing federal laws that have been waived by DHS in furtherance of border construction projects. The report, however, does not discuss proposals to modify this framework or fund an expansion of border fencing. Prior to 1996, federal immigration statutes did not expressly authorize or require the construction of barriers along the U.S. international borders. In the preceding years, authority to deploy any such barriers appears to have primarily derived from the general statutory responsibility of the Attorney General (and now the Secretary of Homeland Security) to "guard the boundaries and borders of the United States against the illegal entry of aliens." Perhaps the most prominent example of this general authority being employed to construct barriers occurred in the early 1990s, when the U.S. Border Patrol (with the assistance of the Department of Defense's Army Corps of Engineers) installed 10-foot-high, welded-steel fencing along roughly 14 miles of the border near San Diego. In 1996, Congress passed IIRIRA, which expressly instructed immigration authorities to construct barriers along the international land borders to deter unauthorized migration. This requirement, contained in IIRIRA Section 102, has been amended on three occasions, and its current language can be viewed at Appendix A . Among other things, IIRIRA Section 102 in its current form generally authorizes DHS to construct barriers and roads along the international borders in order to deter illegal crossings at locations of high illegal entry; requires the construction of reinforced fencing covering at least 700 miles along the southwest border, though the Secretary is not required to install fencing at any particular location; authorizes for the installation of additional physical barriers and infrastructure to gain operational control of the southwest border; requires a specified amount of fencing in priority areas along the southwest border, which DHS was instructed to have completed by December 31, 2008; and provides the Secretary of Homeland Security with authority to waive any legal requirements which may impede construction of barriers and roads under Section 102. The following sections discuss each of these requirements, including how they have been modified over the years. Section 102(a) of IIRIRA provides that the Secretary of Homeland Security "shall take such actions as may be necessary to install additional physical barriers and roads ... in the vicinity of the United States border to deter illegal crossings in areas of high illegal entry into the United States." Although this provision is fashioned as a statutory command, providing that the Secretary "shall" take action, this command is qualified by the language that follows, which affords the Secretary the discretion to determine the appropriate amount of "additional" barriers to deploy, as well as the most appropriate locations to install such barriers "to deter illegal crossings." To the extent that IIRIRA Section 102(a) constitutes a discrete, judicially reviewable command for the Secretary to construct "additional" fencing, immigration authorities seem to have satisfied this mandate by deploying hundreds of miles of additional barriers and roads along the border since IIRIRA was enacted in 1996. Accordingly, this provision perhaps most reasonably could be construed as conferring general authority to the Secretary of Homeland Security to construct barriers and roads along the international borders, so as to deter crossings in areas of "high illegal entry" (a term not defined by the statute). As discussed later in this report, more specific requirements are imposed upon the Secretary by IIRIRA Section 102(b), which requires the Secretary, in exercising the authority conferred under Section 102(a), to ensure that fencing and other barriers are deployed along specified mileage of the southwest border. Section 102(a) generally authorizes the construction of roads and physical barriers, without specifying any particular form that such barriers may take (e.g., reinforced fencing, multilayered fencing, or concrete barriers). Barriers and roads are authorized to be installed along any of the international borders of the United States, at least so long as DHS determines their installation is appropriate to deter unauthorized crossings in areas of high illegal entry. The provision's authorization for the installation of barriers and roads only applies to areas "in the vicinity of the United States border." The phrase "vicinity of the United States border" is not defined under IIRIRA, nor is it described in other federal immigration statutes. As a result, there may be some ambiguity as to the authorized distance from the border where roads and barriers may be constructed. The sole federal court to consider the usage of "vicinity" in IIRIRA Section 102 interpreted the term as including land "situated near the border," rather than only land directly adjacent to the border. Some DHS regulations unrelated to the border fence have described distances up to 25 miles from a location as being within its "vicinity." The Supreme Court, in non-binding dicta, also once characterized a search occurring 25 miles from the border as being within the "general vicinity of the border," though it does not appear that the Court ascribed legal significance to that phrase. There is also no indication in the legislative history of IIRIRA, however, that Congress contemplated the term "vicinity of the United States border" as referring to a specific distance. IIRIRA Section 102(b) imposes specific requirements upon the Secretary of Homeland Security to construct reinforced fencing along the southwest border. The nature of these requirements has changed over the years, including to expand the mileage along the border where fencing must be installed, and to afford the Secretary greater discretion in determining the type of fencing that may be employed and the particular location where fencing shall be installed. In addition to minimum fencing requirements, IIRIRA Section 102(b) authorizes the deployment of "additional physical barriers, roads, lighting, cameras, and sensors to gain operational control of the southwest border." IIRIRA Section 102(b) directed immigration authorities to supplement the already existing 14-mile primary border fence near San Diego with two additional layers of fencing. Environmental concerns and litigation resulted in significant delays in fulfilling this requirement. Over eight years after IIRIRA was enacted, DHS had not completed the fencing project. As discussed later in this report, subsequent expansion of the Secretary of Homeland Security's authority to waive legal requirements that impeded construction of fencing projects, facilitated DHS's efforts to complete a second layer of fencing along the San Diego border. Other amendments made to IIRIRA, discussed below, removed the statutory requirement that DHS complete the San Diego fencing project that had been authorized by IIRIRA when it was originally enacted in 1996. IIRIRA Section 102(b) was substantially revised by the Secure Fence Act of 2006. Section 102(b)'s original requirement concerning fencing in the San Diego area was replaced with a much more expansive instruction to deploy "at least 2 layers of reinforced fencing, [and] the installation of additional physical barriers, roads, lighting, cameras, and sensors" along five specified stretches of the southwest border. CBP estimated that this mandate covered roughly 850 miles. The fencing mandate imposed by the Secure Fence Act was somewhat limited by a specification that "other means" could be used to secure an area where "the topography ... has an elevation grade that exceeds 10 percent.... " In addition to this general mandate, the Secure Fence Act also provided deadlines for the completion of certain border projects. In particular, the act amended IIRIRA Section 102(b) to designate a stretch of border between Calexico, CA, and Douglas, AZ, as a priority area. It directed DHS to ensure that "an interlocking surveillance camera system" would be installed along this area by May 30, 2007, and to provide for the completion of fencing along this stretch by May 30, 2008. A separate 30-mile stretch of fencing near Laredo, TX, was required to be deployed by December 31, 2008. No timetable was specified, however, for DHS to complete double-layered fencing in the remaining stretches of the border. The most recent revisions to IIRIRA Section 102 were enacted slightly more than a year after Congress passed the Secure Fence Act (and prior to the statutory deadlines for the deployment of double-layered fencing under the earlier act). The Consolidated Appropriations Act, 2008 (2008 Appropriations Act) amended IIRIRA Section 102(b) to significantly increase the Secretary of Homeland Security's discretion as to where to construct fencing along the southwest border. In particular, the 2008 Appropriations Act modified IIRIRA Section 102(b) in four ways: Eliminated earlier requirement of double-layered fencing— Whereas the prior language of IIRIRA Section 102(b), as amended by the Secure Fence Act, had generally required "at least 2 layers of reinforced fencing" be deployed in specified areas, Section 102(b) now mandates only a single layer of reinforced fencing (while not precluding additional layers from being deployed, if deemed appropriate). Provided more flexible requirements concerning location of fencing and other border infrastructure— While the Secure Fence Act required fencing to be installed along specific stretches of the southwest border, potentially totaling roughly 850 miles, the 2008 Appropriations Act replaced this specification with a more general requirement that fencing be deployed "along not less than 700 miles of the southwest border where fencing would be most practical and effective." DHS was also instructed to construct "additional physical barriers, roads, lighting, cameras, and sensors to gain operational control of the southwest border." The Appropriations Act also amended IIRIRA Section 102(b) to provide that the Secretary was not obligated to deploy fencing or other border security infrastructure "in a particular location along an international border of the United States, if the Secretary determines that the use or placement of such resources is not the most appropriate means to achieve and maintain operational control over the international border at such location." New deadline for construction of fencing in "priority areas" —The earlier version of IIRIRA Section 102(b) required the construction of fencing along specified stretches of the border, totaling roughly 370 miles, by May 2008, and fencing along another 30-mile section by December 2008. This was replaced with a new requirement that the Secretary of Homeland Security identify 370 miles "or other mileage" along the southwest border where fencing would be "most practical and effective," and complete construction of such fencing by December 31, 2008. New consultation requirements— As amended, Section 102(b) of IIRIRA now requires the Secretary to consult with the Secretaries of the Interior and Agriculture, state and local governments, Indian tribes, and property owners "to minimize the impact on the environment, culture, commerce, and quality of life" in areas near where fencing is to be constructed. The provision further provides that this consultation requirement does not create or negate any right to legal action by an affected person or entity. As noted above, the 2008 Appropriations Act substantially modified IIRIRA Section 102(b) just over a year after the Secure Fence Act had done the same. These revisions, along with sometimes conflicting statements made by DHS officials to Congress concerning the agency's interpretation of its duties under Section 102(b), have potentially contributed to some disagreement regarding the nature of DHS's obligations under IIRIRA Section 102(b). The following paragraphs detail the legislative history of the most recent revisions to IIRIRA Section 102(b), the key elements of the requirements it imposes, and potential constraints on judicial review of implementation of the statute's fencing requirements. The legislative history behind the 2008 Appropriations Act's amendment to IIRIRA Section 102(b) is convoluted, and the explanatory materials for the enacted omnibus do not elaborate upon the amendment's purpose. Nonetheless, materials in the Congressional Record may shed some light as to the sponsors' intended purpose, along with the context in which the proposed amendment was being considered. The modifications to IIRIRA Section 102(b) appear to derive from language originally contained in a floor amendment offered during Senate consideration of the FY2008 homeland security appropriations bill. The floor amendment, offered by Senator Lindsey Graham and co-sponsored by a number of other Senators, would have added a new division entitled the "Border Security First Act of 2007" to the appropriations legislation. In addition to modifying the fencing requirements contained in IIRIRA Section 102(b), the amendment would have required the deployment of 300 miles of vehicle barriers and 700 linear miles of fencing along the U.S.-Mexico border within two years; provided additional resources and requirements for DHS immigration enforcement programs at the border and within the interior of the United States; and made several substantive changes to the Immigration and Nationality Act and the U.S. Criminal Code. Senator Graham and a number of co-sponsors thereafter spoke in favor of the amendment, with three Senators specifically commenting upon the amendment's fencing provisions. Both Senator Graham and Senator Jeff Sessions, an amendment co-sponsor, characterized the amendment as providing for 700 miles of border fencing. Senator John Cornyn, another co-sponsor, highlighted the amendment's proposed change to IIRIRA Section 102(b) as ensuring that DHS consulted with local officials and property owners regarding proposed fencing in a given area. The proposed amendment was later ruled by the Senate chair to be out of order, because some topics of the amendment were deemed non-germane to the appropriations bill being considered. The following day, Senator Graham offered a new amendment that was more limited in scope. The new amendment did not include the original amendment's proposed modification of IIRIRA Section 102(b), or those provisions of the earlier amendment that would have substantively modified federal immigration and criminal statutes. However, the new amendment contained the earlier version's provision requiring DHS to deploy 300 miles of vehicle barriers and 700 linear miles of fencing along the southwest border within two years, along with a related provision appropriating funds for the completion of this project. The amendment to the homeland security appropriations bill was adopted by a vote of 89-1. Separately, Senator Patty Murray offered an amendment on behalf of Senator Kay Bailey Hutchison to revise IIRIRA Section 102(b). The revisions proposed in the Hutchison amendment were for the most part identical to those found in the amendment offered by Senator Graham the previous day. The Hutchison amendment was approved by a voice vote, and both it and the second Graham amendment (which Senator Hutchison also co-sponsored) were included in the Senate-passed version of the homeland security appropriations bill. Later in the year, 11 regular appropriations measures for FY2008, including appropriations for homeland security, were combined into an omnibus bill that became the 2008 Appropriations Act. The omnibus legislation reconciled differences between the competing House- and Senate-passed homeland security appropriations bills. The final act included the provision in the Senate-passed bill that revised IIRIRA Section 102(b), which had been added by the Hutchison amendment. However, the final act did not retain the Senate-passed bill's provisions that had been added by the Graham amendment, which would have separately imposed a two-year deadline for the completion of 700 linear miles of fencing and 300 miles of vehicle barriers, and provided appropriations to ensure that these deadlines were met. The 2008 Appropriations Act amended IIRIRA in a manner that provides DHS with significant discretion as to the manner in which to install fencing—undoing some of the more specific requirements that had previously been imposed. Whereas the Secure Fence Act had amended IIRIRA Section 102(b) to provide that "at least 2 layers of reinforced fencing" were to be installed along specified stretches of the border, the 2008 Appropriations Act replaced this with a more general requirement that "reinforced fencing" be installed along the southwest border. In other words, IIRIRA Section 102(b) no longer requires DHS to install two or more layers of reinforced fencing at any location along the border—a single layer of reinforced fencing appears sufficient to satisfy the statutory mandate. DHS would appear to have discretion to construct additional layers of fencing pursuant to its general authority under IIRIRA Section 102(a), however, if it deems such fencing to be appropriate. Some disagreement has arisen over DHS's use of "vehicle fencing" to satisfy IIRIRA's fencing requirements. Vehicle fencing is a type of barrier designed to inhibit the illegal crossing of vehicles into the United States, but not pedestrians. Some Members of Congress have argued that the "fencing" referred to in IIRIRA Section 102 should be limited to the type that is effective at preventing all illegal entrants, whether traveling by vehicle or by foot. On the other hand, IIRIRA Section 102(b) does not mandate that any particular type of fencing must be deployed, beyond providing that such fencing be "reinforced." The statute does not specify, for example, that deployed fencing must be of a particular height, or be constructed in a particular style (e.g., using bollard, wire mesh, or chain link). In the absence of language specifying the use of a particular kind of fencing, it would appear that DHS enjoys broad discretion to assess the appropriate type of fencing to deploy in order to achieve operational control of the southwest border. The January 25, 2017 executive order issued by President Trump instructs the Secretary of Homeland Security to construct a "physical wall" along the U.S.-Mexico border. The term is defined to refer to a "contiguous, physical wall or other similarly secure, contiguous, and impassable physical barrier ." This definition seems broad enough to include at least some types of fencing, at least so long as such fencing was considered by DHS to be significantly " secure " and " impassable " to be deemed a wall . While IIRIRA Section 102(b) is sometimes characterized as requiring DHS to deploy "700 miles of fencing," the express language of the text seems to impose a slightly different mandate. Section 102(b) requires DHS to deploy fencing "along not less than 700 miles of the southwest border." This seems to be a somewhat different requirement, which prioritizes the actual mileage of the border covered by fencing, rather than just the number of miles of fencing deployed. For example, if DHS hypothetically deployed 30 miles of fencing, but did so through the construction of a 10-layered, 3-mile-long fence, it would have only installed fencing along 3 miles of the border. On the other hand, if DHS deployed such fencing in a single layer of fencing, it would have deployed fencing along 30 miles of the border. Likely because of the phraseology of IIRIRA Section 102(b), DHS seems to only count the mileage of primary layers of fencing deployed along the southwest border when discussing its efforts to satisfy its statutory mandate, and not the total amount of fencing that it has deployed (i.e., including secondary and tertiary layers of fencing that run behind some stretches of primary fencings). As amended by the 2008 Appropriations Act, IIRIRA 102(b) contains two distinct but related mandates. One of the mandates, which can be referred to as the priority fencing mandate , required DHS to identify and complete construction of fencing in priority areas of the southwest border by December 31, 2008. The second mandate, which can be referred to as the general fencing mandate , requires reinforced fencing to be deployed along "not less than 700 miles" of the southwest border, but contains no deadline for deployment. The general fencing mandate is subject to a proviso that DHS is not required to install fencing or other infrastructure "at any particular location" along the border, if the use or placement of fencing at that location is not deemed by the Secretary of Homeland Security to be the most appropriate means to achieve and maintain control. Section 102(b)'s priority fencing mandate required DHS, before December 31, 2008, to identify either 370 miles "or other mileage" along the southwest border where fencing would be most appropriate to deter unlawful migration and smuggling activities. Construction of the identified fencing was required to have also been completed by the end of 2008. Although DHS initially planned to deploy 670 miles of fencing pursuant to this mandate, it subsequently revised these plans prior to the reaching the deadline for priority fence construction. According to a 2010 report by the Government Accountability Office (GAO), DHS opted to comply with the priority fencing mandate by ensuring that reinforced fencing had been deployed along 370 miles of the southwest border. Although the priority fencing mandate of IIRIRA Section 102(b) has been satisfied, the general fencing mandate has not yet been fulfilled. DHS has thus far deployed reinforced fencing along roughly 653 miles of the border. At least on first look, it would appear that the agency would need to install additional fencing along nearly 50 miles of the southwest border to meet the requirements of Section 102(b). In recent years, however, there have been conflicting views among some policymakers as to the firmness of the general fencing mandate . The following section discusses these views. However, any dispute regarding the inflexibility of the 700-mile requirement may be rendered moot if the Trump Administration goes forward with plans to deploy a physical barrier along a greater portion of the U.S.-Mexico border. The rigidity of the requirement that DHS deploy fencing along at least 700 miles of the border turns on the relationship between two clauses in IIRIRA Section 102. Although one clause of IIRIRA Section 102(b) requires fencing "along not less than 700 miles" of the border, another clause in that subsection provides: Notwithstanding [the general fencing mandate of this section] nothing in this paragraph shall require the Secretary of Homeland Security to install fencing, physical barriers, roads, lighting, cameras, and sensors in a particular location along an international border of the United States, if the Secretary determines that the use or placement of such resources is not the most appropriate means to achieve and maintain operational control over the international border at such location. The meaning and effect of this proviso is arguably open to interpretation. One way to read the clause is simply to reflect the discretion that Congress intended to afford DHS in determining where to deploy at least 700 miles of fencing along the southwest border. As discussed previously, prior to amendments made by the 2008 Appropriations Act, the subsection had required DHS to deploy fencing along specific stretches of the southwest border that were identified within the statutory command. The new proviso could be interpreted to emphasize the discretion that DHS was afforded as a result of the revisions made by the 2008 Appropriations Act. While DHS is required to construct fencing along at least 700 miles of the border, the agency retains discretion to determine the most appropriate stretches along the 1,933 mile-land border where the fencing should be deployed. Relatedly, the "notwithstanding" proviso to the general fencing mandate affords a potential response to claims that DHS did not place fencing at a location where it would be "most practical and effective." DHS is not required to install fencing at a location if "the Secretary determines that the placement of such resources is not the most appropriate means to achieve and maintain operational control ... at such location." This is phrased in a manner that suggests that the Secretary's determination not to place fencing at a location would be afforded a very high degree of deference by a reviewing court. In other words, the "notwithstanding" clause enables DHS to readily answer claims by parties seeking to compel some portion of the required border fencing at a location where the fencing would allegedly be "most practical and effective." Arguably, however, the proviso could be interpreted more broadly, to signify something other than simply the discretion that DHS possesses in determining the stretches of the border along which to construct fencing. Although the proviso is not crafted as an express waiver, the clause's usage of "notwithstanding" could be construed to supersede any conflicting requirements imposed by the general fencing mandate . Under a broad interpretation of the "notwithstanding" proviso, it might be argued that the Secretary is not necessarily required to deploy fencing along 700 miles of the border, if the Secretary concludes that fencing is the appropriate means of achieving operational control along a lesser mileage of the border. There are difficulties, however, with interpreting the "notwithstanding" proviso so broadly. As an initial matter, the proviso does not expressly state that DHS may opt to construct a lesser amount of fencing than is specified elsewhere in Section 102(b). Rather, the proviso states that DHS is not required to construct fencing "at a particular location," if the Secretary of Homeland Security determines that the installation of that infrastructure is not appropriate for "such location." As noted above, this provision could reasonably be construed to mean that, in carrying out its general fencing mandate to deploy fencing along 700 miles of the southwest border, DHS is not legally required to install any portion of this required fencing at "any particular location." The ability to construe the proviso in more than one way may be significant. Courts typically follow the interpretive principle that a "statute should be construed so that effect is given to all its provisions, so that no part will be inoperative or superfluous, void or insignificant.... " If Section 102(b)'s proviso is construed to mean that DHS is only required to deploy the amount of fencing along the border that it deems appropriate, the clause would render Section 102(b)'s mandate that fencing be deployed "along at least 700 miles of the border" superfluous. On the other hand, if the proviso is interpreted to mean that, in carrying out its mandate to construct fencing along at least 700 miles of the border the border, DHS is not legally required to install the required fencing at any particular location, every provision of IIRIRA Section 102(b) can be given effect. The legislative history behind IIRIRA Section 102(b)'s fencing requirements also seems to support a narrow construction of Section 102(b)'s "notwithstanding" clause. As discussed previously, the amendments made to IIRIRA Section 102(b) by the 2008 Appropriations Act were originally part of a package of amendments, approved by the Senate as part of a homeland security appropriations bill, which were intended to ensure that fencing along 700 miles of the border would be constructed expeditiously. The legislative history of these amendments' consideration, along with their plain text, seems to indicate that the amendments' purpose was to ensure that fencing was deployed along 700 miles of the border within two years. Indeed, the Senator who offered the amendment to IIRIRA Section 102(b) also co-sponsored these related amendments. While Congress ultimately opted to enact one of these amendments into law but not the other, presumably the Senate would not have originally approved both amendments if they were understood to be conflicting. To date, it appears that every federal court which has discussed IIRIRA Section 102(b) has described the provision in mandatory terms: DHS is required to deploy fencing along 700 miles of the southwest border, but it retains discretion to determine the appropriate locations in which to deploy the required fencing. It should be noted, however, that no court has definitively ruled that an alternative interpretation is not permissible, or closely examined the interplay between the "notwithstanding" proviso and the general fencing requirement. But the uniform interpretation may suggest that, as a matter of first impression, Section 102(b) may be most reasonably construed as establishing a mandate to deploy fencing along at least 700 miles of the border. For its part, DHS has appeared to take conflicting views regarding the firmness of IIRIRA Section 102(b)'s general fencing mandate . Initially, DHS appeared to construe the 700-mile specification as a firm requirement. In notices issued in the Federal Register in 2008 describing border fencing projects undertaken under IIRIRA Section 102(b), Secretary of Homeland Security Michael Chertoff stated that "Congress has called for the installation of fencing, barriers, roads, lighting, cameras, and sensors on not less than 700 miles of the southwest border …." In March 2009, Secretary of Homeland Security Janet Napolitano wrote to the House Homeland Security Committee in response to questions regarding DHS's obligations to deploy fencing along the southwest border. Secretary Napolitano described IIRIRA as mandating that DHS construct at least 700 miles of fencing, but at least for the immediate future, DHS would focus on fence deployment in priority areas: As amended, the Act mandates the completion of 700 total miles of fence. It also mandates that the Secretary identify priority areas "where fencing would be the most practical and effective in deterring smugglers and aliens attempting to gain illegal entry into the United States." As of March 6, 2009, DHS has completed approximately 611 of the 661 miles of fence identified by the Border Patrol as priority areas. While fencing remains an important tool in achieving effective control, it is only one element of our overall border security strategy that incorporates the proper mix of technology, personnel, and tactical infrastructure. Currently, there are no immediate funded plans to construct additional fencing. DHS later appeared to modify its interpretation of the IIRIRA Section 102(b), and began to describe the "notwithstanding" proviso as permitting it to deploy fencing along less than 700 miles of the border, if the agency deemed a lesser amount of fencing to be appropriate to achieve operational control. Indeed, four years after describing IIRIRA Section 102(b) as imposing a firm mandate, Secretary Napolitano gave testimony before the Senate Judiciary Committee in which she appeared to take the view that DHS was legally permitted to construct a lesser amount of fencing. To date, DHS has not publicly released a formal legal opinion describing its interpretation of the fencing mandate established by IIRIRA Section 102(b), or announced whether or how its opinion has changed over the years. In defending DHS against a legal challenge by the state of Arizona in 2011, in which Arizona sought to compel DHS to complete construction of fencing along 700 miles of the border (and undertake other immigration enforcement actions), the Department of Justice did not dispute the existence of this mandate, but instead argued that DHS decisions as to where to locate such fencing and the speed by which fencing was to deployed were committed to agency discretion. It should be noted that, in assessing the permissibility of an agency's interpretation of the laws it administers, reviewing courts typically accord the agency's interpretation of these statutes with some degree of deference, so long as the construction is reasonable. In determining whether an agency's construction of a statute is reasonable, legislative intent is a touchstone for a reviewing court's analysis—an agency's interpretation might be entitled to deference when congressional intent is ambiguous and the agency's construction of the statute is reasonable. Moreover, agency interpretations of statutory requirements are usually afforded a lesser degree of deference when the agency interpretation is not the result of a notice-and-comment rulemaking process or formal adjudication. In such circumstances, the level of deference given to the agency's interpretation "will depend upon the thoroughness evident in its consideration, the validity of its reasoning, its consistency with earlier and later pronouncements, and all those factors which give it power to persuade, if lacking power to control." The appropriate degree of deference to which DHS's interpretation of the statutory requirements imposed by IIRIRA should be afforded may be subject to differing views. Assuming that DHS's interpretation of the requirements under IIRIRA Section 102(b) is subject to legal challenge, the degree of deference that a reviewing court gives to the agency's interpretation may be informed by a number of factors, including whether (1) the plain text of the statute is ambiguous, and DHS's interpretation is reasonable; (2) other indicia of legislative intent favor a particular interpretation; and (3) the agency's apparent modification of its interpretation of IIRIRA Section 102(b) entitles its current interpretation to a lesser degree of deference. Regardless of the appropriate interpretation of Section 102(b)'s general fencing mandate and the "notwithstanding" proviso, the statute imposes no clear deadline for when the contemplated fencing must be deployed. In the 2011 litigation in which Arizona sought to compel DHS to complete construction of fencing required under IIRIRA Section 102(b), the reviewing federal district court dismissed Arizona's motion, in part because "no deadline mandates completion of the fencing and infrastructure developments or any required discrete action by a specified time." The court further observed that, although "the construction of the fencing and infrastructure improvements may be phrased in mandatory language," IIRIRA affords DHS "a great deal of discretion in deciding how, when, and where to complete the construction." The absence of a deadline for the completion of the fencing requirements of IIRIRA Section 102(b) does not necessarily mean that DHS has no judicially enforceable legal obligation to complete any remaining fencing that is statutorily required. The Administrative Procedure Act (APA), for example, provides courts with the authority to compel agency action, when such action has been "unlawfully withheld or unreasonably delayed." Determining whether an agency has unreasonably delayed undertaking a required action is a fact-specific determination made on a case-by-case basis, with reviewing courts typically showing more deference to an agency when there is not a statutory deadline for agency action. If a court determined that DHS had unreasonably delayed fulfillment of its obligations under IIRIRA Section 102(b), it might deem the completion of at least 700 miles of fence along the southwest border to constitute "a discrete agency action" that it would potentially have the power to compel. The district court in the Arizona case found that completion of the border fence was not a "discrete agency action" that it could compel DHS to take, but it did not explain the basis for this conclusion. Moreover, even assuming that a court might have jurisdiction to review a claim that DHS has unreasonably delayed fence construction, it is not clear who would have standing to make such a claim. To demonstrate standing, a plaintiff must (1) show a concrete and particularized, actual or imminent injury; (2) demonstrate a fairly traceable, causal connection between the injury and the defendant's conduct; and (3) demonstrate that it is likely that the injury will be redressed by a favorable court decision. It may be difficult for a plaintiff to identify a concrete, particularized injury that would be effectively remedied if DHS deployed fencing along an additional 50 miles of the border. In the Arizona case, the district court presumed without deciding that Arizona had standing but dismissed its claims. Section 102(c) of IIRIRA confers the Secretary of Homeland Security with broad authority to waive legal requirements that may impede the construction of barriers and roads along the border. The nature and scope of this waiver authority changed significantly pursuant to modifications made by the REAL ID Act of 2005. In the years following, the Secretary of Homeland Security employed this waiver authority to facilitate the construction of hundreds of miles of fencing and other infrastructure along several sections of the southwest border. More recently, however, this waiver authority has not been employed to facilitate further large-scale border projects. President Trump's executive order calling for immediate construction of additional physical barriers along the U.S.-Mexico border did not directly call for the invocation of this waiver authority by the Secretary, but such authority could be employed at a later date to facilitate the deployment of a border wall. When initially enacted in 1996, IIRIRA Section 102(c) expressly authorized the waiver the Endangered Species Act (ESA) and the National Environmental Policy Act (NEPA), to the extent that such waivers were determined necessary by the Attorney General to expeditiously construct barriers and roads under Section 102. Other federal laws, however, remained applicable to border construction projects. Federal immigration authorities appear to have not employed IIRIRA Section 102(c), as originally enacted, to waive NEPA and ESA requirements. In 2004, eight years after IIRIRA mandated completion of a second and third layer of fencing along the San Diego border, fencing was still not completed due to litigation and concerns related to environmental statutes other than NEPA and the ESA. The California Coastal Commission essentially halted the San Diego fencing project after determining that DHS had not demonstrated, among other things, that the project was "consistent to the maximum extent practicable" with the policies of the California Coastal Management Program—a state program approved under the federal Coastal Zone Management Act, 16 U.S.C. §§1451, et seq . In part due to delays in the construction of fencing near San Diego, Congress amended IIRIRA Section 102(c) via the REAL ID Act of 2005. As amended, IIRIRA Section 102(c) permits the Secretary of Homeland Security to waive "all legal requirements" necessary to ensure expeditious construction of these security barriers. Such waivers are effective upon publication in the Federal Register . Federal district courts are provided with exclusive jurisdiction to review claims alleging that the actions or decisions of the Secretary violate the U.S. Constitution, and district court rulings may be reviewed only by the Supreme Court, whose review is discretionary. The scope of this waiver authority is substantial—with some observers describing it as possibly having greater reach than any other waiver authority conferred by statute —leading some to express concern over its breadth and the limited scope of judicial review available for waiver decisions. Although IIRIRA Section 102(c), as amended by the REAL ID Act, could not properly be construed to permit the Secretary to waive application of the Constitution to fencing projects, the waiver potentially could be employed with respect to any other existing legal requirement—provided that the Secretary of Homeland Security concluded that compliance with the requirement would impede expeditious construction of barriers and roads. Waivers issued under IIRIRA Section 102(c) have not only targeted federal and state statutes, but also any regulations and requirements deriving from or relating to such laws. Nonetheless, the waiver authority conferred by IIRIRA Section 102(c) is not absolute. The Secretary may only waive those legal requirements that, in effect, would impede the construction of barriers and roads under Section 102. The authority does not appear to permit the Secretary to waive legal requirements that only tangentially relate to, or do not necessarily interfere with, the construction of roads and barriers. The decision of whether to waive a legal requirement is the responsibility of the Secretary of Homeland Security, and authority may be exercised in his discretion. Until such time as the Secretary waives an applicable law, however, DHS must generally follow all legal requirements normally imposed on federal agencies. To date, the Secretary of Homeland Security has provided notice in the Federal Register on five occasions that he was invoking the waiver authority conferred under IIRIRA Section 102(c): San Diego Border Sector— On September 22, 2005, a notice was issued in the Federal Register indicating that waiver authority had been exercised over various legal requirements in order to ensure the expeditious construction of the San Diego border fence. The waiver applies to "all federal, state, or other laws, regulations and legal requirements of, deriving from, or related to the subject," of various federal statutes listed in Appendix B . Barry M. Goldwater Range (BMGR) in Southwestern Arizona— A Federal Register notice was published on January 19, 2007, indicating that the Secretary was waiving various legal requirements in order to ensure the expeditious construction of physical barriers and roads in the vicinity of the BMGR in southwestern Arizona. The waiver applies to all "[f]ederal, [s]tate, or other laws, regulations and legal requirements of, deriving from, or related to the subject" of several federal statutes listed in Appendix C . San Pedro Riparian National Conservation Area in Southeastern Arizona —On October 5, 2007, Defenders of Wildlife and the Sierra Club brought suit seeking a temporary restraining order (TRO) enjoining DHS from border fence and road-building activities in the San Pedro Riparian National Conservation Area, located in the vicinity of the U.S. border in southeastern Arizona. On October 10, 2007, the presiding district court judge issued a TRO halting fence construction activities in the Conservation Area, finding the relevant federal agencies had failed to carry out an environmental assessment as legally required under NEPA. On October 26, 2007, a notice was published in the Federal Register indicating that the Secretary of Homeland Security had exercised waiver authority over various legal requirements in order to ensure the expeditious construction of physical barriers or roads through the San Pedro Riparian National Conservation Area (including any and all lands covered by the TRO), thereby enabling DHS to resume fence construction. The waiver applies to "all federal, state, or other laws, regulations and legal requirements of, deriving from, or related to the subject" of a collection of federal statutes listed in Appendix D . Hidalgo County, Texas— On April 3, 2008, notice was given in the Federal Register that the Secretary of Homeland Security had exercised his waiver authority under IIRIRA Section 102(c) to ensure the construction of barriers and roads in Hidalgo County, Texas. The waiver applies to "all federal, state, or other laws, regulations and legal requirements of, deriving from, or related to the subject" of various federal statutes listed in Appendix E . Border Projects in California, Arizona, New Mexico, and Texas— On April 3, 2008, the Secretary of Homeland Security gave notice in the Federal Register of the waiver of various laws in relation to border construction projects in California, Arizona, New Mexico, and Texas. The waiver applies to "all federal, state, or other laws, regulations and legal requirements of, deriving from, or related to the subject" of various federal statutes listed in Appendix F . In multiple instances, lawsuits were brought challenging the constitutionality of an issued waiver. Constitutional claims raised in these collective cases included arguments that the waiver authority was an impermissible delegation of Congress's lawmaking authority; the waiver provision violated the Presentment Clause by effectively enabling the executive branch to "repeal" or "amend" existing laws to exempt border infrastructure projects from their coverage; and the waiver authority's application to state and local laws violated federalism and anti-commandeering principles. In each case, the reviewing federal district court upheld the exercise of waiver authority as constitutionally valid. Parties in two of the cases sought Supreme Court review, but the Court declined to grant certiorari in either case. Pursuant to IIRIRA Section 102, Congress has conferred DHS with clear authority to construct barriers and roads along the international land borders to deter illegal crossings in areas of high illegal entry. More specifically, it has required fencing to be constructed along specified mileage of the southwest border. In recent years, legislative attention has primarily focused upon the fencing requirements contained in IIRIRA Section 102(b). Prior versions of Section 102(b) imposed specific requirements as to the location where fencing was to be installed and the layers of fencing to be constructed. The current provision affords DHS with significantly greater discretion to determine the appropriate location, layers, and types of fencing to be installed along the southwest border. Whether DHS has discretion to construct less fencing than the amount specified under IIRIRA Section 102(b), on account of a proviso that posits that the agency is not required to construct fencing at any "particular location" where it deems fencing to be inappropriate, has been the subject of disagreement (and apparently inconsistent views by DHS itself). While there appears to be stronger support for construing Section 102(b) to establish a firm mandate for the deployment of fencing along 700 miles of the border, with the agency retaining discretion as to the locations along the border where fencing should be installed, it is not clear whether a court would have the ability to compel DHS to install additional fencing (or that a plaintiff would have standing to bring such a claim). If Congress disagrees with DHS's implementation of the fencing mandate under Section 102(b), it would likely need to enact legislation to modify or clarify the fencing requirements found in current statute. But even assuming that DHS satisfies the fencing requirements under Section 102(b), the general authority conferred to the agency under Section 102(a) permits it to construct additional fencing or other barriers along the U.S. land borders. Moreover, Section 102(b) authorizes DHS to deploy additional physical barriers—beyond the mandated fencing along 700 miles of the southwest border—in order to obtain operational control of the southwest border. There is nothing in current statute that would appear to bar DHS from potentially installing hundreds of miles of additional fencing or other physical barriers along the border, at least so long as the action was determined appropriate to deter illegal crossings in areas of high illegal entry. In addition, IIRIRA Section 102(c) grants DHS authority to waive any legal requirement that would impede the expeditious construction of additional barriers and roads. DHS's decision not to deploy a substantial amount of additional fencing, beyond what is required under IIRIRA Section 102(b), appears primarily premised on policy considerations and funding constraints, rather than significant legal impediments. Accordingly, policymakers may deem it appropriate to review and assess the scope of DHS's authority to construct barriers, and the manner in which such authority is exercised, even after any requirements under IIRIRA Section 102(b) are satisfied. Appendix A. IIRIRA Section 102, as Amended (Text) Sec. 102 - Improvement of Barriers at Border (a) In General.-The Secretary of Homeland Security shall take such actions as may be necessary to install additional physical barriers and roads (including the removal of obstacles to detection of illegal entrants) in the vicinity of the United States border to deter illegal crossings in areas of high illegal entry into the United States. (b) Construction of Fencing and Road Improvements Along the Border.- (1) Additional fencing along southwest border.- (A) Reinforced fencing.-In carrying out subsection (a), the Secretary of Homeland Security shall construct reinforced fencing along not less than 700 miles of the southwest border where fencing would be most practical and effective and provide for the installation of additional physical barriers, roads, lighting, cameras, and sensors to gain operational control of the southwest border. (B) Priority areas.-In carrying out this section [amending this section], the Secretary of Homeland Security shall- (i) identify the 370 miles, or other mileage determined by the Secretary, whose authority to determine other mileage shall expire on December 31, 2008, along the southwest border where fencing would be most practical and effective in deterring smugglers and aliens attempting to gain illegal entry into the United States; and (ii) not later than December 31, 2008, complete construction of reinforced fencing along the miles identified under clause (i). (C) Consultation.- (i) In general.-In carrying out this section, the Secretary of Homeland Security shall consult with the Secretary of the Interior, the Secretary of Agriculture, States, local governments, Indian tribes, and property owners in the United States to minimize the impact on the environment, culture, commerce, and quality of life for the communities and residents located near the sites at which such fencing is to be constructed. (ii) Savings provision.-Nothing in this subparagraph may be construed to- (I) create or negate any right of action for a State, local government, or other person or entity affected by this subsection; or (II) affect the eminent domain laws of the United States or of any State. (D) Limitation on requirements.-Notwithstanding subparagraph (A), nothing in this paragraph shall require the Secretary of Homeland Security to install fencing, physical barriers, roads, lighting, cameras, and sensors in a particular location along an international border of the United States, if the Secretary determines that the use or placement of such resources is not the most appropriate means to achieve and maintain operational control over the international border at such location. (2) Prompt acquisition of necessary easements.-The Attorney General, acting under the authority conferred in section 103(b) of the Immigration and Nationality Act [8 U.S.C. 1103(b)] (as inserted by subsection (d)), shall promptly acquire such easements as may be necessary to carry out this subsection and shall commence construction of fences immediately following such acquisition (or conclusion of portions thereof). (3) Safety features.-The Attorney General, while constructing the additional fencing under this subsection, shall incorporate such safety features into the design of the fence system as are necessary to ensure the well-being of border patrol agents deployed within or in near proximity to the system. (4) Authorization of appropriations.-There are authorized to be appropriated such sums as may be necessary to carry out this subsection. Amounts appropriated under this paragraph are authorized to remain available until expended. (c) Waiver.- (1) In general.-Notwithstanding any other provision of law, the Secretary of Homeland Security shall have the authority to waive all legal requirements such Secretary, in such Secretary's sole discretion, determines necessary to ensure expeditious construction of the barriers and roads under this section [amending this section]. Any such decision by the Secretary shall be effective upon being published in the Federal Register. (2) Federal court review.- (A) In general.-The district courts of the United States shall have exclusive jurisdiction to hear all causes or claims arising from any action undertaken, or any decision made, by the Secretary of Homeland Security pursuant to paragraph (1). A cause of action or claim may only be brought alleging a violation of the Constitution of the United States. The court shall not have jurisdiction to hear any claim not specified in this subparagraph. (B) Time for filing of complaint.-Any cause or claim brought pursuant to subparagraph (A) shall be filed not later than 60 days after the date of the action or decision made by the Secretary of Homeland Security. A claim shall be barred unless it is filed within the time specified. (C) Ability to seek appellate review.-An interlocutory or final judgment, decree, or order of the district court may be reviewed only upon petition for a writ of certiorari to the Supreme Court of the United States. Appendix B. Legal Requirements Waived by DHS for the Construction of the San Diego Border Fence Appendix C. Legal Requirements Waived by DHS for the Construction of Physical Barriers and Roads in the Vicinity of the Barry M. Goldwater Range in Southwest Arizona Appendix D. Legal Requirements Waived by DHS for the Construction of Physical Barriers and Roads in the Vicinity of the San Pedro Riparian National Conservation Area in Southeast Arizona Appendix E. Legal Requirements Waived by DHS for the Construction of Physical Barriers and Roads in Hidalgo County, Texas Appendix F. Legal Requirements Waived by DHS for the Construction of Physical Barriers and Roads at Various Project Areas Located in California, Arizona, New Mexico, and Texas

Summary: Federal law authorizes the Department of Homeland Security (DHS) to construct barriers along the U.S. borders to deter illegal crossings. DHS is also required to construct reinforced fencing along at least 700 miles of the land border with Mexico (a border that stretches 1,933 miles). Congress has not provided a deadline for DHS to meet this 700-mile requirement, and as of the date of this report, fencing would need to be deployed along nearly 50 additional miles to satisfy the 700-mile requirement. Nor has Congress provided guidelines regarding the specific characteristics of fencing or other physical barriers (e.g., their height or material composition) deployed along the border, beyond specifying that required fencing must be reinforced. The primary statute authorizing the deployment of fencing and other barriers along the international borders is Section 102 of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA; P.L. 104-208, div. C). Congress made significant amendments to IIRIRA Section 102 through three enactments—the REAL ID Act of 2005 (P.L. 109-13, div. B), the Secure Fence Act of 2006 (P.L. 109-367), and the Consolidated Appropriations Act, 2008 (P.L. 110-161, div. E). These amendments required DHS to construct hundreds of miles of new fencing along the U.S.-Mexico border, and they also gave the Secretary of DHS broad authority to waive "all legal requirements" that may impede construction of barriers and roads under IIRIRA Section 102. These statutory modifications, along with increased funding for border projects, resulted in the deployment of several hundred miles of new barriers along the southwest border between 2005 and 2011. But in the years following, DHS largely stopped deploying additional fencing, as the agency altered its enforcement strategy in a manner that places less priority upon barrier construction. On January 25, 2017, President Donald J. Trump issued an executive order that, among other things, instructs the Secretary of Homeland Security to "take all appropriate steps to immediately plan, design, and construct a physical wall along the southern border ... to most effectively achieve complete operational control" of the U.S.-Mexico border. The order defines a "wall" to mean "a contiguous, physical wall or other similarly secure, contiguous, and impassable physical barrier." The order does not identify the contemplated mileage of the wall to be constructed. Until recently, interest in the framework governing the deployment of barriers along the international border typically focused on the stringency of the statutory mandate to deploy fencing along at least 700 miles of the U.S.-Mexico border. But attention has now shifted to those provisions of law that permit deployment of fencing or other physical barriers along additional mileage. IIRIRA Section 102 authorizes DHS to construct additional fencing or other barriers along the U.S. land borders beyond the 700 miles specified in statute. Indeed, nothing in current law would appear to bar DHS from installing hundreds of miles of additional physical barriers, at least so long as this action was determined appropriate to deter illegal crossings in areas of high illegal entry or was deemed warranted to achieve "operational control" of the southern border. DHS's policy not to deploy a substantial amount of additional fencing, beyond what is expressly required by law, appeared primarily premised on policy considerations and funding constraints, rather than significant legal impediments. This report discusses the statutory framework governing the deployment of fencing and other barriers along the U.S. international borders. For more extensive discussion of ongoing activities and operations along the border between ports of entry, see CRS Report R42138, Border Security: Immigration Enforcement Between Ports of Entry, by [author name scrubbed].