The country's immigration and naturalization laws have been subjects of episodic controversy since America's founding, but unauthorized migration only became an issue in the early 20 th century, when Congress passed the first strict restrictions on legal admissions. Unauthorized migration declined during the Great Depression and during and after World War II, when most labor migration occurred through the U.S.-Mexico Bracero program. Immigration control re-emerged as a national concern during the 1970s, when the end of the Bracero program, new restrictions on Western Hemisphere migration, and growing U.S. demand for foreign-born workers combined to cause a sharp increase in unauthorized migration flows. Congress responded in 1986 by passing the Immigration Reform and Control Act (IRCA, P.L. 99-603 ), which authorized a 50% increase in Border Patrol staffing, among other provisions. Border security has remained a persistent topic of congressional interest since then, and enforcement programs and appropriations have grown accordingly, as described in this report. Despite a growing enforcement response, however, unauthorized migration continued to increase over most of the next three decades. After 2005, unauthorized migrant apprehensions began to decline, suggesting a decrease in unauthorized migration. Apprehensions of unauthorized migrants at the U.S.-Mexico border fell from about 1.2 million in FY2005 to a 41-year low of 378,577 in 2011. Apprehensions then climbed to 479,371 in FY2014, before falling to 331,333 in FY2015. Additionally, it is estimated that after 2007 the number of unauthorized migrants living in the United States also declined. In 2014, the Obama Administration announced executive actions to "fix" the immigration system. These actions address several issues, including a security plan at the southern border. Some Members of Congress and state officials, however, disagree with the President's plan and have called on the Administration to do more to secure the border. Border security has been a recurrent theme in Congress's debate about immigration reform since 2005, and some Members of Congress have argued that Congress should not consider additional immigration reforms until the border is more secure. This report reviews efforts to combat unauthorized migration across the Southwest border in the nearly three decades since IRCA initiated the modern era in migration control. In reviewing such efforts, the report takes stock of the current state of border security and considers lessons that may be learned about enhanced enforcement at U.S. borders. The report begins by reviewing the history of border control and the development of a national border control strategy beginning in the 1990s. The following sections summarize appropriations and resources dedicated to border enforcement, indicators of enforcement outcomes, metrics for border security, and possible secondary and unintended consequences of border enforcement. The report concludes by reviewing the overall costs and benefits of the current approach to migration control and raising additional questions that may help guide the discussion of these issues in the future. Congress created the U.S. Border Patrol (USBP) within the Departments of Commerce and Labor by an appropriations act in 1924, two days after passing the first permanent numeric immigration restrictions. Numerical limits only applied to the Eastern Hemisphere, barring most Asian immigration; and the Border Patrol's initial focus was on preventing the entry of Chinese migrants, as well as combating gun trafficking and alcohol imports during prohibition. The majority of agents were stationed on the northern border. The Border Patrol became part of the new Immigration and Naturalization Service (INS) in 1933, and the INS moved from the Department of Labor to the Department of Justice in 1940. The Border Patrol's focus shifted to the Southwest border during World War II, but preventing unauthorized migration across the Southwest border remained a low priority during most of the 20 th century. Unauthorized migration from Mexico increased after 1965 as legislative changes restricted legal Mexican immigration at the same time that social and economic changes caused stronger migration "pushes" in Mexico (e.g., inadequate employment opportunities) and stronger "pulls" in the United States (e.g., employment opportunities, links to migrant communities in Mexico). Congress held hearings on unauthorized migration beginning in 1971, and after more than a decade of debate passed the Immigration Reform and Control Act of 1986 (IRCA, P.L. 99-603 ), which described border enforcement as an "essential element" of immigration control and authorized a 50% increase in funding for the Border Patrol, among other provisions. Congress passed at least 11 additional laws addressing unauthorized migration over the next two decades, 7 of which included provisions related to the border. Seventy years after it began operations, the Border Patrol developed its first formal national border control strategy in 1994, the National Strategic Plan. The plan was updated in 2004 and again in 2012. The National Strategic Plan (NSP) was developed in 1994 in response to a widespread perception that the Southwest border was being overrun by unauthorized migration and drug smuggling, and to respond to a study commissioned by the Office of National Drug Control Policy. The study recommended that the INS change its approach from arresting unauthorized migrants after they enter the United States, as had previously been the case, to focus instead on preventing their entry. Under the new approach, the INS would place personnel, surveillance technology, fencing, and other infrastructure directly on the border to discourage unauthorized flows, a strategy that became known as "prevention through deterrence." According to the 1994 INS plan, "the prediction is that with traditional entry and smuggling routes disrupted, illegal traffic will be deterred, or forced over more hostile terrain, less suited for crossing and more suited for enforcement." The strategy had four phases that began with the border patrol sectors with the highest levels of unauthorized migration activity. Phase I: San Diego, CA, and El Paso, TX, sectors; Phase II: Tucson, AZ, Del Rio, TX, Laredo, TX, and McAllen, TX, sectors; Phase III: El Centro, CA, Yuma, AZ, and Marfa, TX, sectors; and Phase IV: The northern border, gulf coast, and coastal waterways. The strategy yielded several initiatives aimed at stemming unauthorized migration and human smuggling, and interdicting drug trafficking. These initiatives included the following: Operation Gatekeeper was first initiated at the San Diego Border Patrol Sector and was later extended to the El Centro Border Patrol Sector. The initiative involved providing border patrol agents with additional resources, such as increased staffing and new technologies. Operation Safeguard was initiated at the Tucson Border Patrol Sector and was aimed at stemming unauthorized migration by pushing unauthorized migrants away from urban areas. Operation Hold the Line was initiated at the El Paso Border Patrol Sector and was aimed at the specific needs of the community. New border patrol agents were added to the area and innovative resources were deployed, including IDENT terminals. The initiative also included the installation of fences along parts of the border and other infrastructure improvements. Operation Rio Grande was initiated at the McAllen Border Patrol Sector and focused on increasing the number of border patrol agents. The implementation of Phase II and subsequent phases was to be based on the success of Phase I, with the plan describing several expected indicators of effective border enforcement, including an initial increase of border arrests and entry attempt to be followed by an eventual reduction of arrests, a change in traditional traffic patterns, and an increase in more sophisticated smuggling methods. As predicted, apprehensions within the San Diego and El Paso sectors fell sharply beginning in 1994-1995, and traffic patterns shifted, primarily to the Tucson and South Texas (Rio Grande Valley) sectors (see " Southwest Border Apprehensions by Sector "). A 1997 General Accounting Office (GAO, now called the Government Accountability Office) report was cautiously optimistic about the strategy. Congress supported the prevention through deterrence approach. In 1996, House and Senate appropriators directed the INS to hire new agents and to reallocate personnel from the interior to front line duty. And the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 ( P.L. 104-208 ) expressly authorized the construction and improvement of fencing and other barriers along the Southwest border and required the completion of a triple-layered fence along 14 miles of the border near San Diego where the INS had begun to install fencing in 1990. In the wake of the September 11, 2001, terrorist attacks, the USBP refocused its priorities on preventing terrorist penetration, while remaining committed to its traditional duties of preventing the illicit trafficking of people and contraband between official ports of entry. Shortly after the creation of DHS, the USBP was directed to formulate a new National Border Patrol Strategy (NBPS) that would better reflect the realities of the post-9/11 security landscape. In March 2004, the Border Patrol unveiled the National Border Patrol Strategy, which placed greater emphasis on interdicting terrorists and featured five main objectives: establishing the substantial probability of apprehending terrorists and their weapons as they attempt to enter illegally between the ports of entry; deterring unauthorized entries through improved enforcement; detecting, apprehending, and deterring smugglers of humans, drugs, and other contraband; leveraging "Smart Border" technology to multiply the deterrent and enforcement effect of agents; and reducing crime in border communities, thereby improving the quality of life and economic vitality of those areas. The NBPS was an attempt to lay the foundation for achieving "operational control" over the border, defined by the Border Patrol as "the ability to detect, respond, and interdict border penetrations in areas deemed as high priority for threat potential or other national security objectives." The strategy emphasized a hierarchical and vertical command structure, featuring a direct chain of command from headquarters to the field. The document emphasized the use of tactical, operational, and strategic intelligence and sophisticated surveillance systems to assess risk and target enforcement efforts; and the rapid deployment of USBP agents to respond to emerging threats. Additionally, the plan called for the Border Patrol to coordinate closely with CBP's Office of Intelligence and other federal intelligence agencies.