Congress and the Donald J. Trump Administration are debating enhancing and expanding barriers on the southwest border. The extent of these barriers, and how construction of these barriers will be funded has become a central part of the interactions between Congress and the Trump Administration on border security and funding legislation for the broader federal government. The debate has revealed the lack of an authoritative compilation of data on the details of federal investment in border barriers. This is in part due to the evolving structure of the appropriations for agencies charged with protecting the borderâaccount structures have shifted, initiatives have come and gone, and appropriations prior to FY2017 typically did not specify a precise level of funding for barriers as opposed to other technologies that secure the border. The Trump Administration's continued advocacy for funding for a "border wall system" has led to a congressional interest in the historical context for border barrier funding. This report briefly contextualizes the history of U.S. enforcement of the U.S.-Mexico border, before turning to funding for border barriers within the contemporary period, accounting for changing appropriations structures. The Treaty of Guadalupe Hidalgo in 1848, with the cession of land to the United States, ended the Mexican-American War and set forth an agreed-upon boundary line between the United States and Mexico. The physical demarcation of the boundary was essentially set by the Gadsden Purchase, finalized in 1854, with some minor adjustments since then. Securing U.S. borders has primarily been the mission of the U.S. Border Patrol, which was established by Congress by an appropriations act in 1924. Initially, a relatively small force of 450 officers patrolled both the northern and southern borders between inspection stations, guarding against the smuggling of contraband and unauthorized migrants. The Immigration Act of 1924 established immigration quotas for most countries, with the exception of those in the Western Hemisphere, including Mexico. (While some specific limitations existed, per-country quotas for Western Hemisphere countries did not exist until 1976. ) Earlier policies had set categorical exclusions to entry (e.g., for Chinese and other Asian immigrants) that were exceptions to an otherwise open immigration policy. Between 1942 and 1964, the Bracero Program brought in nearly 5 million Mexican agricultural workers to fill the labor gap caused by World War II. Both employers and employees became used to the seasonal work, and when the program ended, many continued this employment arrangement without legal authorization. Debates about enhancing enforcement of immigration laws ensued in the late 1970s and 1980s, largely in concert with counter-drug smuggling efforts and interest in curbing the rise in unauthorized flows of migrant workers. A significant effort to construct barriers on the southern border as a deterrent to illegal entry by migrants or smugglers into the United States began in the early 1990s. In 1991, U.S. Navy engineers built a ten-foot-high corrugated steel barrier between San Diego and Tijuana made of surplus aircraft landing mats, an upgrade to the previous chain-link fencing. In 1994, the Border Patrol (then part of the Department of Justice under the Immigration and Naturalization Service, INS) released a strategic plan for enforcing immigration laws along the U.S. border, as a part of a series of immigration reform initiatives. The plan, developed by Chief Patrol Agents, Border Patrol headquarters staff, and planning experts from the Department of Defense Center for Low Intensity Conflict, described their approach to improving control of the border through a strategy of "prevention through deterrence," under which resources were concentrated in major entry corridors to establish control of those areas and force traffic to more difficult crossing areas. The Border Patrol will increase the number of agents on the line and make effective use of technology, raising the risk of apprehension high enough to be an effective deterrent. Because the deterrent effect of apprehensions does not become effective in stopping the flow until apprehensions approach 100 percent of those attempting entry, the strategic objective is to maximize the apprehension rate. Although a 100 percent apprehension rate is an unrealistic goal, we believe we can achieve a rate of apprehensions sufficiently high to raise the risk of apprehension to the point that many will consider it futile to continue to attempt illegal entry. Prior to 1996, federal statute neither explicitly authorized nor required barrier construction along international borders. In 1996, the Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA) was enacted, and Section 102(a) specifically directed the Attorney General 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." Following the terrorist attacks of September 11, 2001, the U.S. government changed its approach to homeland security issues, including control of the border. As a part of the establishment of the Department of Homeland Security (DHS) in 2003, INS was dismantled, and the Border Patrol and its responsibility for border security were moved from the Department of Justice to DHS as a part of U.S. Customs and Border Protection (CBP). DHS and CBP were stood up in 2003, and received their first annual appropriations in FY2004. During the 109 th and the first session of the 110 th Congresses (2005-2007), comprehensive immigration reform legislation and narrower border security measures were debated. One result was that Congress explicitly authorized and funded new construction of border barriers, significantly increasing their presence. In the 109 th Congress, two bills were enacted that amended Section 102 of IIRIRA, easing the construction of additional border barriers. Section 102 of the REAL ID Act of 2005 ( P.L. 109-13 , Div. B) included broad waiver authority that allowed for expedited construction of border barriers. The Secure Fence Act of 2006 ( P.L. 109-367 ) directed the Secretary of Homeland Security to "achieve and maintain operational control over the entire international land and maritime borders of the United States," mandated the construction of certain border barriers and technology on the border with Mexico by the end of 2008, and required annual reports on progress on border control. This was a different approach in border barrier legislation. Past immigration policy bills had included border barriers as a part of a suite of remedies across government to the border security problem in the context of immigration policy. The Secure Fence Act instead provided authorization for DHS alone to achieve "operational control" of the border through barriers, tactical infrastructure, and surveillance while largely not addressing the broader set of immigration policies that could contribute to improved border security. In addition, the Secure Fence Act substantially revised IIRIRA Section 102(b) to include five specific border areas to be covered by the installation of fencing, additional barriers, and technology. The FY2006 DHS Appropriations Act ( P.L. 109-90 ) provided the first appropriations specifically designated for the Border Patrol (now under CBP and a part of DHS) to construct border barriers. The act specified $35 million for CBP's San Diego sector fencing. This funding was part of a surge in CBP construction spending from $91.7 million in FY2005âand $93.4 million in the FY2006 requestâto $270.0 million for FY2006 enacted appropriations. This direction also represented the first specific statutory direction provided to CBP on the use of its construction funds. Toward the end of 2007, Congress amended Section 102 of IIRIRA through Section 564 of the Consolidated Appropriations Act, 2008. Congress again required the construction of reinforced fencing along at least 700 miles of the U.S.-Mexico border, where it would be "most practical and effective," but also included flexibility in implementing this requirement, stating that: 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. Starting in FY2007 and continuing through FY2016, border barrier funding in CBP's budget was included in the "Border Security Fencing, Infrastructure, and Technology" (BSFIT) appropriation. When BSFIT was established in the Department of Homeland Security Appropriations Act, 2007 ( P.L. 109-295 ), it consolidated border technology and tactical infrastructure funding from other accounts, including CBP's Construction appropriation and Salaries and Expenses appropriation. According to the FY2007 DHS appropriations conference report, Congress provided $1,512,565,000 for BSFIT activities for FY2007: $1,187,565,000 from annual appropriations in P.L. 109-295 , and $325,000,000 in prior enacted supplemental appropriations from P.L. 109-234 and other legislation. Congress directed portions of that initial appropriation to two specific border security projects, and withheld $950 million until a spending plan for a border barrier was provided. Starting in FY2008, a PPA for "Development and Deployment" of technology and tactical infrastructure was included at congressional direction under the BSFIT appropriation. The BSFIT Development and Deployment PPA is, over the tenure of CBP, the most consistently structured year-to-year direction from Congress to CBP regarding putting border security technology and infrastructure in the field, covering FY2008-FY2016. The BSFIT Development and Deployment structure remained unchanged until the implementation of the Common Appropriations Structure (CAS) for DHS in the FY2017 appropriations cycle, which redistributed BSFIT funding to the Operations and Support (OS) appropriation and the Procurement, Construction, and Improvements (PC&I) appropriation. Border barrier design and construction funding, other than ports of entry, is now included in the Border Security Assets and Infrastructure PPA along with several other activities. Figure 1 shows the requested and enacted levels for the Development and Deployment PPA from FY2008 through FY2016. Although it doesn't show an almost $1.2 billion FY2007 appropriation for border infrastructure before the Development and Deployment PPA was implemented, it does indicate the early high levels of investment in border infrastructure, which then tapered off. The dashed line shows the size of the budget request for these elements. While the new structure of appropriations made it clear that funding was being directed to border security enhancements, the level of detail was not always sufficient to identify the funding level for barrier construction. CRS was able to obtain this more granular information directly from CBP, which provided a breakdown to CRS of its spending on border barriers beginning with FY2007. The primary program that funded barrier construction in this period was the Tactical Infrastructure (TI) Program. Figure 2 and Table 1 present funding data provided by CBP for border barriers under the TI program. The funding provided in FY2007 to FY2009 resulted in increased border barrier construction (which extended for a few years into the early 2010s). As the funds for construction were expended, CBP transitioned its border barrier activities to primarily maintenance and minor repairs, until FY2017. CBP has indicated in follow-up communications that no further historical data are available, as barrier construction was conducted by several entities within CBP, and not centrally tracked. In addition, the definitions of tactical infrastructure may allow for inclusion of some elements only peripherally related to border barriers. Taking these factors into account, given the limited mileage constructed prior to FY2007 (see Appendix for details), the above data present the best available understanding of appropriations and spending on border barriers in the 2007-2016 period. On January 25, 2017, the Trump Administration issued Executive Order 13767, "Border Security and Immigration Enforcement Improvements." Section 2(a) of the EO indicates that it is the policy of the executive branch to "secure the southern border of the United States through the immediate construction of a physical wall on the southern border, monitored and supported by adequate personnel so as to prevent illegal immigration, drug and human trafficking, and acts of terrorism." The EO goes on to define "wall" as "a contiguous, physical wall or other similarly secure, contiguous, and impassable physical barrier." For FY2017, changes were made both in the structure of how funds were appropriated, and how CBP organized those funds among its authorized activities. This complicates efforts to make detailed comparisons in funding levels between the present and time periods prior to FY2016. When DHS was established in 2003, components of other agencies were brought together over a matter of months, in the midst of ongoing budget cycles. Rather than developing a new structure of appropriations for the entire department, Congress and the Executive continued to provide resources through existing account structures when possible. CBP's budget structure evolved over the DHS's early years, including the institution of the Border Security Fencing, Infrastructure, and Technology (BSFIT) account in FY2007. At the direction of Congress, in 2014 DHS began to work on a new Common Appropriations Structure (CAS), which would standardize the format of DHS appropriations across components. After several years of negotiations with Congress, DHS made its first budget request in the CAS for FY2017, and implemented the new budget structure while operating under a continuing resolution in October 2016. This resulted in the BSFIT structure being eliminated. The funding that had been provided under its appropriation would now be provided under the CBP Operations and Support (OS) and Procurement, Construction, and Improvements (PC&I) appropriations. Aside from the appropriations structure, changes within CBP's internal account structure occurred during FY2017. The "Wall Program" was established at CBP during FY2017. The Wall Program is a lower-level PPA nested within the new Border Security Assets and Infrastructure activity, which in turn is a part of the CBP PC&I appropriation. According to CBP, the Wall Program oversees the execution of the FY2017 TI program funding and "will be responsible for all future wall construction." CBP first directed appropriations to the Wall Program in FY2018 ($1.375 billion). CBP's TI Program continues to manage the funding for maintenance of new and replacement border barriers, as it has since FY2007. Table 2 shows appropriations for border barriers requested by the Administration and provided by Congress in the DHS appropriations acts. Each fiscal year is discussed in detail after Table 2 . The Trump Administration submitted a supplemental appropriations request in March 2017 for a variety of priorities, including CBP staffing and border wall construction. The request for additional CBP PC&I funding included $1.38 billion, of which $999 million was for "planning, design, and construction of the first installment of the border wall." The FY2017 DHS Appropriations bill included a sixth title with the congressional response to the supplemental appropriations request. It included $341.2 million to replace approximately 40 miles of existing primary pedestrian and vehicle barriers along the southwest border "using previously deployed and operationally effective designs, such as currently deployed steel bollard designs, that prioritize agent safety" and to add gates to existing barriers. The Administration requested $1.72 billion for the Border Security Assets and Infrastructure PPA, including $1.57 million for construction of border barriers. In the FY2018 appropriations measure, Congress provided $1.74 billion, which, according to a House Appropriations Committee summary, included funding for "over 90 miles of physical barrier construction along the southern borderâincluding replacement, bollards, and levee improvements." Section 230 of the bill specified the following $1.375 billion for the following activities under the CBP PC&I appropriation: $445 million for 25 miles of primary pedestrian levee fencing in Rio Grande Valley (RGV) sector; $196 million for primary pedestrian fencing in RGV sector; $251 million for secondary replacement fencing in San Diego sector; $445 million for replacement of existing primary pedestrian fencing; and $38 million for border barrier planning and design. The section went on to note that the funding for primary fencing "shall only be available for operationally effective designs deployed as of [May 5, 2017], such as currently deployed steel bollard designs that prioritize agent safety." The Administration initially requested $1.647 billion for the Border Security Assets and Infrastructure PPA. Budget justification documents noted that $1.6 billion was requested for the border wall. The Administration reportedly requested $5.0 billion for the wall from Republican congressional leadership. However, no publicly available modification of its request was presented to Congress until January 6, 2019. At that time, in the midst of a lapse in annual appropriations due in part to conflict over border barrier funding, the acting head of the Office of Management and Budget (OMB) submitted a letter seeking $7 billion in additional border related funding. The $7 billion included $4.1 billion more for "the wall" than the Administration originally requested. The letter indicated that the total request of $5.7 billion would pay for "approximately 234 miles of new physical barrier and fully fund the top 10 priorities in CBP's Border Security Improvement Plan." P.L. 116-6 , the Consolidated Appropriations Act, 2019, included $1.375 billion for CBP "for the construction of primary pedestrian fencing, including levee pedestrian fencing, in the Rio Grande Valley Sector." Funding could only be used for "operationally effective designs deployed as of [May 5, 2017], such as currently deployed steel bollard designs that prioritize agent safety." The same day that the President signed the FY2019 consolidated appropriations act into law, he declared a national emergency on the southern border of the United States. A fact sheet accompanying the declaration indicated the President's intent to make additional funding available for border barriers through three methods, sequentially. These methods and related actions are: 1. Drawing about $601 million from the Treasury Forfeiture Fund A letter from the Department of the Treasury on February 15, 2019, indicated that those funds would be made available to DHS for "law enforcement border security efforts" ($242 million available March 2, and $359 million after additional forfeitures were received). According to court documents, Treasury transferred the full $601 million to DHS on September 27, 2019. CBP will reportedly use the funds as follows: $261 million for future-year real estate planning and acquisition for border barrier construction along the southwest border. $340 million for border barrier projects in the Rio Grande Valley Sector, of which $124 million is for construction; and $216 million is for construction management costs, increased project costs, and real estate planning and acquisition. 2. Making up to $2.5 billion available through the Department of Defense's support for counterdrug activities (authorized under 10 U.S.C. Â§284) $1 billion has been reprogrammed within the Department of Defense to its Drug Interdiction and Counter Drug Activities account, and that funding, in turn, was transferred for the U.S. Army Corps of Engineers to do certain DHS-requested work on border barriers. On May 10, 2019, the Department of Defense announced an additional $1.5 billion reprogramming of funding that had been dedicated to a variety of initiatives, including training and equipping Afghan security forces, programs to dismantle chemical weapons, and other activity for which savings or program delays had been identified. The DOD indicated that the funding would construct an additional 80 miles of border barriers. Use of both of these tranches of reprogrammed funds to pay for border barrier projects had been blocked by a court injunction until July 26, 2019, when the Supreme Court ruled that the government could proceed with the use of the funds while a lower court determines the legality of the transfer that made the funds available. 3. Reallocating up to $3.6 billion from various military construction projects under the authority invoked by the emergency declaration On September 3, 2019, Secretary of Defense Mark Esper issued a memorandum with the determination that "11 military construction projects â¦ along the international border with Mexico, with an estimated total cost of $3.6 billion, are necessary to support the use of the armed forces in connection with the national emergency [at the southern border]." The memorandum indicates $1.8 billion in unobligated military construction funding for overseas projects would be made available immediately, while $1.8 billion in domestic military construction projects would be provided once it is needed. In February 2019, The Administration requested $5 billion in border barrier funding for FY2020, to support the construction of approximately 206 miles of border wall system. The House Appropriations Committee included no funding for border barriers when it reported its FY2020 DHS appropriations bill. In addition, the bill would have restricted the ability to transfer or reprogram funds for border barrier construction and proposed rescinding $601 million from funding appropriated for border barriers in FY2019. The Senate Appropriations Committee took the opposite approach when it reported S. 2582 , recommending $5 billion for border barrier construction. It also did not include any of the House bill's proposed restrictions or the rescission. Neither the House nor the Senate considered these appropriations bills on the floor. The FY2020 DHS Appropriations Act ( P.L. 116-93 , Div. D)âwhich was passed as part of the Consolidated Appropriations Act, 2020âincluded $1.375 billion for "construction of barrier system along the southwest border." The barrier system design restrictions are similar to prior years, with a new exception for designs that help "mitigate community or environmental impacts." There is an additional requirement that the barriers are to be built in the highest priority locations identified in CBP's Border Security Improvement Plan. Figure 3 presents a comparison of the total funding made available in the first and second eras of DHS efforts to support planning and construction of barriers on the U.S.-Mexico border. This comparison is made with two important caveats: the data sources and funding structures are different in the two eras. In the legislative era (FY2007-FY2016), detailed information was provided directly to CRS in a communication from CBP. It was tracked for "tactical infrastructure," which included funding for border roads and other TI. In the executive era (FY2017 to the present), data from CBP and appropriations measures (which has been more detailed with respect to barrier planning and construction) are generally consistent, but the Administration uses the specifically defined "border wall" program to track most of the funding. A small amount of funding for barrier replacement and supporting infrastructure was provided through the tactical infrastructure PPA in FY2018. Section 4 of E.O. 13767, "Physical Security of the Southern Border of the United States," focuses almost entirely on the construction of "a physical wall" on the U.S.-Mexico border as a means of obtaining operational control of the nearly 2,000-mile border. CBP has indicated that it cannot provide authoritative historical data prior to FY2007 on the level of funding invested in border barrier planning and construction. To briefly recap the funding that has been provided by Congress in response to the Trump Administration's initiative, the $4.47 billion in appropriations provided by Congress to CBP for border barrier planning and construction during the Trump Administration exceeds the amount provided for those purposes in the BSFIT account for the 10 years from FY2007 to FY2016 by $2 billion. Of the $4.47 billion: $1.04 billion was specifically directed to barrier replacement projects; $2.02 billion was specifically directed to construction needs in the Rio Grande Valley Sector; and $1.41 billion has been provided for planning and construction of border barriers without specific direction in regards to location or whether the funding was for barrier replacement or construction of additional miles of barriers. Despite the historically high volume of resources provided, the Administration has taken unprecedented stepsânoted aboveâin an attempt to more than double the funding level appropriated to CBP by Congress for barrier construction since the signing of E.O. 13767. $601 million was provided to DHS in FY2019 from the Treasury Forfeiture Fund. As noted in " Border Barrier Funding Outside the Appropriations Process ," $124 million of that funding is being used for construction, while $477 million is for real estate planning and acquisition, increased project costs, and construction management costs. Generally, the Administration, in its discussion about border barriers, relies on the U.S. Border Patrol Impedance and Denial Prioritization Strategy , which includes a list of projects for barrier construction. There are no known authoritative cost estimates for the total construction or operation and maintenance costs of these projects if they are all completed, or publicly available assessments of how completion of various projects might affect CBP's operational costs. Furthermore, GAO reported in 2016 that the border barriers' contributions to CBP goals were not being adequately measured. GAO reported in 2018 that CBP's methodology for prioritizing border barrier deployments did not use cost estimates that included data on topography, land ownership, and other factors that could impact the costs of individual barrier projects. The Administration's stated intent is to expand the amount of border barriers on the southwest border, and this issue will likely be part of debates on the budgets for the current and future fiscal years. Congress may wish to obtain the following information and explore the following questions in assessing border barrier funding proposals: 1. What are the projected operation and maintenance costs for the existing southwest border barriers? How will those change with additional replacements, upgrades, or new construction of barriers? 2. What are the projected land acquisition and construction costs of CBP's remaining top priority border barrier projects, based on unique topography, land ownership, and strategic intent of the projects? What steps is CBP taking to control the growth of those costs? Who within the Administration is providing oversight of how these funds are used, and are they reporting their findings to Congress? 3. Are existing barriers and completed improvements having measurable impacts on attempted illegal entry into the U.S. and smuggling of contraband? How are CBP and other stakeholders making their assessments? Is CBP getting its desired tactical or strategic outcomes? 4. Are the operational benefits worth the financial and operational costs, or are there more efficient ways to achieve the desired tactical or strategic outcomes? 5. How should Congress respond to the Administration's exercise of reprogramming and transfer authorities to provide funding for border barrier work above the amount Congress provided to CBP for that purpose? The United States' southern border with Mexico runs for nearly 2,000 miles over diverse terrain, through varied population densities, and across discontinuous sections of public, private, and tribal land ownership. The Department of Homeland Security (DHS) Customs and Border Protection (CBP) is primarily responsible for border security, including the construction and maintenance of tactical infrastructure, but also the installation and monitoring of surveillance technology, and the deployment of border patrol agents to impede unlawful entries of people and contraband into the United States (e.g., unauthorized migrants, terrorists, firearms, and narcotics). Built barriers, such as fencing, are a relatively new feature on the southern border. These structures vary in age, purpose, form, and location. At the end of FY2015, approximately 653 milesâroughly one-third of the international boundaryâhad a primary layer of barriers. Approximately 300 miles of the "primary fence" was designed to prevent vehicles from entering, and approximately 350 additional miles was designed to block people on footâ"pedestrian fencing." CBP has used various materials for pedestrian fencing, including bollard, steel mesh, and chain link, and employed bollard and Normandy-style fencing for vehicle barriers. Across 37 discontinuous miles, the primary layer is backed by a secondary layer of pedestrian fencing as well as an additional 14 miles of tertiary fencing (typically to delineate property lines). On January 10, 2020, the Administration announced the completion of the first 100 miles of the "new border wall system" under the Trump Administration. Based on CBP's information, the 100 miles of new border wall system largely replaces less formidable existing barriers with 18- to 30-foot bollard style fencing designed to obstruct both vehicles and pedestrians. It does not represent additional miles of the primary layer of border barriers. CBP has not announced the completion of any additional miles of primary fencing since 2015, but sections of legacy fencing and breached areas have been replaced or repaired and other improvements have been made. An interactive online project by inewsource (a nonprofit, nonpartisan investigative online newsroom in San Diego) and KPBS (a Public Broadcasting Service television and radio station in San Diego, California) used data obtained via a Freedom of Information Act (FOIA) request to Customs and Border Protection to account for every mile of existing border fencing by the year built. The data are used in this appendix to produce Figure A-1 showing the number of miles of primary border barrier constructed for the period 1960-2018 (annual data shown in Table A-1 ). Small areas of the border had fencing prior to 1990. By 1993, fencing in the San Diego area had been completed, covering the first 14 miles of the border east from the Pacific Ocean and a few other areas. Under the provisions of IIRIRA, the Secretary of Homeland Securityâand, prior to 2003, the Attorney Generalâhas the discretion to determine the appropriate amount of additional barriers to build, as well as their location. Approximately 40 additional miles of primary fence were constructed on the southern border through 2005. The vast majority of the existing primary barriersâmore than 525 milesâwere constructed between 2007 and 2009 (see Table A-1 and Figure A-1 ).

Summary: The purpose of barriers on the U.S.-Mexico border has evolved over time. In the late 19 th and early 20 th centuries, fencing at the border was more for demarcation, or discouraging livestock from wandering over the border, rather than deterring smugglers or illegal migration. Physical barriers to deter migrants are a relatively new part of the border landscape, first being built in the 1990s in conjunction with counterdrug efforts. This phase of construction, extending into the 2000s, was largely driven by legislative initiatives. Specific authorization for border barriers was provided in 1996 in the Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA), and again in 2006 in the Secure Fence Act. These authorities were superseded by legislation included in the Consolidated Appropriations Act, 2008, which rewrote key provisions of IIRIRA and replaced most of the Secure Fence Act. The result of these initiatives was construction of more than 650 miles of barriers along the nearly 2,000-mile border. The Trump Administration has driven the second phase of construction of border barriers. On January 25, 2017, the Administration issued Executive Order 13767, "Border Security and Immigration Enforcement Improvements." Section 2(a) of the E.O. indicates that it is the policy of the executive branch to "secure the southern border of the United States through the immediate construction of a physical wall on the southern border, monitored and supported by adequate personnel so as to prevent illegal immigration, drug and human trafficking, and acts of terrorism." The debate over funding for and construction of a "border wall system" in this phase has created congressional interest in the historical context of border barrier funding. There has not been an authoritative compilation of data on the level of federal investment in border barriers over time. This is in part due to the evolving structure of the appropriations for agencies charged with protecting the borderâaccount structures have shifted, initiatives have come and gone, and appropriations typically have not specified a precise level of funding for barriers as opposed to other technologies that secure the border. Funding was not specifically designated for border barrier construction until FY2006. The nearly $4.5 billion in appropriations provided by Congress for border barrier planning and construction since the signing of the E.O. exceeds the amount provided for those purposes from FY2007 to FY2016 combined by almost $2 billion. Most of the contracts that have been awarded thus far are for improvements to, or replacements of, the existing barriers at the border. However, a significant portion of the funds appropriated to the Department of Homeland Security (DHS) is available for construction of barriers where they do not currently exist. The Administration took steps in FY2019 to secure funding beyond the levels approved by Congress for border barriers. These included: transferring roughly $601 million from the Treasury Forfeiture Fund to U.S. Customs and Border Protection (CBP); using $2.5 billion in Department of Defense funds transferred to the Department's counterdrug programs to construct border barriers; and reallocating up to $3.6 billion from other military construction projects using authorities under the declaration of a national emergency. This report provides an overview of the funding appropriated for border barriers, based on data from CBP and congressional documents, and a primer on the Trump Administration's efforts to enhance the funding for border barriers, with a brief discussion of the legislative and historical context of construction of barriers at the U.S-Mexico border. It concludes with a number of unanswered questions Congress may wish to explore as this debate continues. An appendix tracks reported barrier construction mileage on the U.S.-Mexico border by year.