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. 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. 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. In November 2005, the Department of Homeland Security announced a comprehensive multi-year plan, the Secure Border Initiative (SBI), to secure U.S. borders and reduce unauthorized migration. Surveillance technology . Tactical infrastructure . CBP Consequence Delivery System

Although not the subject of a formal public policy document like those discussed above, an additional component of CBP's approach to border control in recent years has been an effort to promote "high consequence" enforcement for unauthorized Mexicans apprehended at the border. This section reviews trends in each of these areas. Falling apprehensions may reflect fewer unauthorized inflows between 2006 and 2012, though the degree to which reduced inflows were a result of effective enforcement versus other factors like the recent U.S. economic downturn remains subject to debate (see " How Secure is the U.S. Border? The agency has used these data since 2006 to inform tactical decision making and to allocate resources across Southwest border sectors, but the Border Patrol has not published them or viewed them as reliable metrics of border security because of challenges associated with measuring got aways and turn backs across different border sectors. Border security metrics are used at both the strategic and operational levels. First, the percentage of people apprehended multiple times along the Southwest border, or the recidivism rate, is used to capture USBP's ability to deter migrants from re-entering the United States. While no single metric accurately and reliably describes border security, most analysts agree, based on available data, that the number of unauthorized border crossers fell sharply between about 2005 and 2011, with some rise in unauthorized flows from 2012 to 2014, and a decrease in 2015. Disentangling the effects of enforcement from other factors influencing migration flows is particularly difficult in the current case because many of the most significant new enforcement efforts—including a sizeable share of new border enforcement personnel, most border fencing, new enforcement practices at the border, and many of the new migration enforcement measures within the United States—all have occurred at the same time as the most severe recession since the 1930s. Thus, to the extent that border enforcement successfully deters unauthorized flows, enforcement benefits the environment by reducing these undesirable outcomes. For these reasons, successful border enforcement may benefit border communities by reducing unauthorized inflows. Yet immigration enforcement occasionally has been a source of bilateral tension. Conclusion: Understanding the Costs and Benefits of Border Enforcement between Ports of Entry

The United States has focused substantial resources along its land borders to prevent and control unauthorized migration since the 1980s, with investments in personnel, fencing, and surveillance assets all up significantly in the post 9/11 period, in particular. On the other hand, there is also some evidence that migrants have adapted to more difficult conditions at the border by using other means to enter the United States and by remaining longer. In the context of immigration policy and a possible immigration reform bill, Members of Congress may choose to focus on the total number of unauthorized migrants in the United States, in addition to border flows, since border enforcement is just one of many factors (along with interior enforcement, visa policies, etc.)
Border enforcement is a core element of the Department of Homeland Security's effort to control unauthorized migration, with the U.S. Border Patrol (USBP) within the U.S. Customs and Border Protection (CBP) as the lead agency along most of the border. Border enforcement has been an ongoing subject of congressional interest since the 1970s, when unauthorized immigration to the United States first registered as a serious national problem; and border security has received additional attention in the years since the terrorist attacks of 2001.
Since the 1990s, migration control at the border has been guided by a strategy of "prevention through deterrence"—the idea that the concentration of personnel, infrastructure, and surveillance technology along heavily trafficked regions of the border will discourage unauthorized migrants from attempting to enter the United States. Since 2005, CBP has attempted to discourage repeat unauthorized migrant entries and disrupt migrant smuggling networks by imposing tougher penalties against certain unauthorized migrants, a set of policies eventually described as "enforcement with consequences." Most people apprehended at the Southwest border are now subject to "high consequence" enforcement outcomes.
Across a variety of indicators, the United States has substantially expanded border enforcement resources over the last three decades. Particularly since 2001, such increases include border security appropriations, personnel, fencing and infrastructure, and surveillance technology. In addition to increased resources, the USBP has implemented several strategies over the past several decades in an attempt to thwart unauthorized migration. In 2014, the Obama Administration announced executive actions to "fix" the immigration system. These actions address several issues, including a revised security plan at the southern border.
The Border Patrol collects data on several different border enforcement outcomes; this report describes trends in border apprehensions, recidivism, and estimated "got aways" and "turn backs." Yet none of these existing data are designed to measure unauthorized border flows or the degree to which the border is secured. Thus, the report also describes methods for estimating border security at the strategic and operational levels.
Drawing on multiple data sources, the report reviews the state of border security. Robust investments at the border were not associated with reduced unauthorized inflows during the 1980s and 1990s, but a range of evidence suggests a substantial drop in unauthorized inflows from 2007 to 2011, followed by a rise from 2012 to 2014 and a decrease in 2015. Enforcement, along with the 2007 economic downturn in the United States, likely contributed to the drop in unauthorized migration, though the precise share of the decline attributable to enforcement is unknown.
Enhanced border enforcement also may have contributed to a number of secondary costs and benefits. To the extent that border enforcement successfully deters unauthorized entries, such enforcement may reduce border-area violence and migrant deaths, protect fragile border ecosystems, and improve the quality of life in border communities. But to the extent that migrants are not deterred, the concentration of enforcement resources on the border may increase border area violence and migrant deaths, encourage unauthorized migrants to find new ways to enter and to remain in the United States for longer periods of time, damage border ecosystems, harm border-area businesses and the quality of life in border communities, and strain U.S. relations with Mexico and Canada.