How the U.S. Patrols Its Borders
Border crossings have continued to fall since Trump reentered office in January 2025. As of May, encounters at the southwestern border for FY 2025 totaled just slightly over four hundred thousand. However, experts say unauthorized immigration has been dropping since early 2024—well before Trump’s inauguration—due to various factors. These include stronger border enforcement, the expansion of legal immigration pathways under Biden, and the Biden administration’s attempts to restrict asylum applications. Mexican authorities also increased efforts to slow U.S.-bound migration and increased deportations. This downward trend in migration has been furthered by the Trump administration, which has taken action to tighten immigration policies and enforcement measures.
Where Are Most Border Crossings Taking Place?
The southern U.S. border, stretching nearly two thousand miles from southern California to the southern tip of Texas at the Gulf of Mexico, has long been the area of greatest concern for Border Patrol agents. Of the nine border sectors spread across Arizona, California, New Mexico, and Texas, the greatest number of encounters in 2024 occurred along the western part of the border, in Arizona’s Tucson Sector. This was followed by the San Diego Sector, in southern California. Previously, most migrant encounters took place in Texas’s Rio Grande Valley and Del Rio Sectors.
The demographics and origins of migrants arriving at the southern border have varied over time. For much of the 1990s and 2000s, they were largely from Mexico and often adults in search of work. Although migration from Mexico fell sharply beginning in the mid-2000s, it continues to be the primary country of origin for entrants. Meanwhile, the number of migrants from other regions, including Asia and Central and South America, has risen in recent years.
Who Is Responsible for U.S. Border Security?
Securing the borders primarily falls to U.S. Customs and Border Protection (CBP), a branch of DHS. Alongside agencies such as the Transportation Security Administration and Drug Enforcement Administration (DEA), CBP is responsible for overseeing and enforcing laws related to trade and travel in and out of the country. Its duties include preventing criminals, would-be terrorists, and contraband from entry. CBP inspects immigrants and cargo at 328 official ports of entry, patrols thousands of miles of border to the country’s north and south, and helps investigate criminal networks, among other responsibilities [PDF]. Of CBP’s more than sixty thousand employees, some one-third are Border Patrol agents, who exclusively work between ports of entry.
When Are National Guard forces Deployed to the Border?
The National Guard, a reserve military force deployed for a wide range of missions at home and abroad, has been called on several times over the past two decades to assist border agents with unauthorized immigration and drug trafficking. National Guard soldiers can be called to action by either a state governor or, in some cases, the president. The Bush administration deployed roughly 6,000 National Guard troops to the border, and the Obama administration sent about 1,200 before trimming down the force.
In 2018, the Biden administration deployed guard members along the southern border as part of a joint operation with CBP known as Guardian Support. Their mission was to assist border agents with logistics, administrative duties, surveillance, and intelligence analysis, as well as provide aerial and mechanical support. By August 2020, the Pentagon had sent more than 2,500 National Guard members [PDF] to aid CBP in Arizona, California, New Mexico, and Texas. The first Trump administration kept National Guard troops there through 2020, even as apprehensions of migrants plummeted and other National Guard personnel were deployed to assist with the pandemic response. In mid-2021, the Biden administration announced that as many as three thousand military personnel would assist CBP and DHS until September 2022.
Texas alone has a significant number of National Guard troops at its southern border. Since 2021, Governor Greg Abbott has implemented Operation Lone Star, a border enforcement program that aims to curb illegal border crossings with the help of the Texas National Guard and state troopers. Part of the plan included deploying floating barriers in the Rio Grande to dissuade migrants from crossing. It also called for the installation of razor wire along the Texas-Mexico border, a move that the Supreme Court later blocked. The operation has so far resulted in more than five hundred thousand apprehensions and nearly forty-nine thousand criminal arrests. Despite the program’s price tag exceeding $11 billion, it has received support from more than two dozen Republican governors.
When Are Active-Duty Troops Deployed There?
It has historically been rare for active-duty U.S. military forces to be sent to the border. In recent decades, soldiers have at times coordinated with border authorities to provide high-tech surveillance and other reconnaissance.
In late 2018, the Pentagon sent more than five thousand troops to “harden the southern border,” employing them for efforts such as laying razor wire. Critics called the move a political stunt amid a midterm election cycle in which Trump made immigration a central issue.
Since then, the number of active-duty troops assigned to the southern border has varied. In April 2020, the Pentagon sent roughly 540 additional active-duty personnel to provide surveillance and prevent migrants from entering the United States amid the pandemic. And in May 2023, ahead of the expiration of a pandemic-era health restriction known as Title 42, the Biden administration temporarily deployed an additional 1,500 active-duty troops to the border to relieve pressure on CBP agents.
The Defense Department maintained a presence of active-duty troops at the border through 2024, as migration levels remained high. Since reentering office in January 2025, Trump has increased the number of troops stationed at the border from some 2,500 to approximately 8,500.
What Are the Rules of Engagement?
The 1878 Posse Comitatus Act limits the U.S. military’s role in enforcing domestic laws, restricting interactions between active-duty troops and migrants at the U.S.-Mexico border. Under these rules, active-duty troops can neither detain and deport unauthorized immigrants nor conduct searches and seizures, though loopholes exist. Like the National Guard, they often provide indirect support, such as conducting aerial surveillance, repairing or reinforcing infrastructure, and performing administrative duties. Unlike the National Guard and CBP, however, active-duty personnel at the border do not carry loaded weapons.
Armed officials are generally constrained from using deadly force. Under CBP policy [PDF], agents are allowed to use force considered “objectively reasonable and necessary” to gain control of a situation, taking into consideration whether a person poses a security threat or is resisting arrest; excessive force is prohibited. An agent may use deadly force only in the case of imminent danger of death or serious injury. In November 2018, Trump also allowed active-duty troops to conduct crowd control and temporary searches and seizures to assist CBP agents.
CBP personnel face hundreds of assaults each year. In FY 2023, nearly five hundred officers and agents were attacked while on duty at the southern border, and close to two hundred such incidents have already occurred in the first seven months of FY 2024. The annual number of incidents involving use of force by CBP personnel rose steadily between 2017 and 2021, though it has since dropped; 304 people—both citizens and noncitizens—have been reported killed in confrontations with border agents since 2010.
CBP personnel face hundreds of assaults each year. In FY 2024, just about four hundred officers and agents were attacked while on duty at the southern border, and more than two hundred such incidents have already occurred in the first eight months of FY 2025. Incidents involving use of force by CBP personnel rose steadily between 2017 and 2021 as migrant encounters surged, though they have since dropped. A total of 341 people—both citizens and noncitizens—have been reported killed as a result of encounters with border agents since January 2010.
What Approach Did the First Trump Administration Take?
Trump repeatedly framed border enforcement as a national security priority, warning of Central American gang members and would-be South Asian and Middle Eastern terrorists joining caravans of migrants to infiltrate the United States. In response to rising migration, Trump declared a national emergency in 2019—which he extended the following year—that allowed him to redirect some $10 billion from the military budget to fund construction of a wall along the U.S.-Mexico border. He argued the wall was necessary to keep out criminals and halt the flow of illegal drugs.
The Trump administration also pursued several policies to block or otherwise discourage asylum seekers from coming to the United States.
Zero tolerance. With long waits at ports of entry driving more asylum seekers to cross at unauthorized points, the administration implemented a blanket zero-tolerance policy [PDF] intended to increase prosecutions of undocumented migrants who crossed the border illegally, including asylum seekers. (Such prosecutions were relatively infrequent under Presidents Bush and Obama.)
The administration also used the policy to separate children from their parents and detain minors; although Trump signed an executive order in 2018 to end family separations, they continued. The administration also sought to narrow the criteria for asylum [PDF] and deny migrants the right to seek asylum outside ports of entry, but federal courts blocked both of these efforts.
Migrant Protection Protocols. In 2019, a new policy called the Migrant Protection Protocols—also known as the “Remain in Mexico” program—began requiring migrants to stay in Mexico while U.S. courts processed their asylum claims. The Mexican government agreed to offer them visas and work permits while they waited. Some migrants applied for asylum in Mexico, but they frequently faced unsafe conditions, and the country’s bureaucracy was stretched beyond its capacity to process claims.
Third-country agreements. The administration sought to strike “safe third country” agreements with several Latin American countries. These agreements required that migrants seek asylum in the first safe country they enter, effectively barring them from applying for asylum in the United States. While the Trump administration signed deals with El Salvador, Guatemala, and Honduras in 2019, only the agreement with Guatemala was implemented; it was later terminated in 2021.
Pandemic restrictions. Claiming asylum became even more difficult during the pandemic after the Trump administration invoked Title 42, a public health statute that allowed the government to turn away migrants to prevent the possible entry of communicable diseases. Critics, however, said the statute violated the United States’ international obligations under the 1951 Refugee Convention, which guarantees the right to seek asylum.
How Did Biden Respond?
During Biden’s presidency, experts and lawmakers grew increasingly concerned with the link between rising immigration—with border crossings reaching a record high in 2023—and the spread of illegal narcotics. This primarily included the synthetic opioid fentanyl, for which part of the drug’s supply chain runs through Mexico.
As part of his immigration agenda, Biden dismantled several Trump-era policies while preserving others. He initially suspended federal construction of the southern border wall before moving forward with plans to build additional sections, and he rescinded Trump’s national emergency declaration. The administration also ended Trump’s zero-tolerance policy and the controversial “Remain in Mexico” program. Ahead of the expiration of Title 42 in May 2023, the administration implemented a restrictive new policy that allowed the government to deny asylum on certain conditions.
At the same time, to incentivize legal migration, the Biden administration expanded legal pathways for migrants to apply for asylum. These included creating humanitarian parole programs for migrants from certain countries, including Afghanistan and Ukraine; opening new screening centers in several Latin American countries, such as Colombia and Guatemala; and building upon migration-related pledges made at the 2022 Summit of the Americas.
Still, the administration faced criticism from some lawmakers who argued the situation had become the “worst border crisis in U.S. history.” Ahead of the 2024 presidential campaign, Biden embraced more restrictive border measures aimed at curbing migrant arrivals. These included fast-tracked asylum screenings and the implementation of a controversial new policy that temporarily blocked people who illegally crossed the U.S.-Mexico border from seeking asylum once the number of daily crossings reached a certain threshold.
How Is the Second Trump Administration Addressing Border Security?
Trump has again made securing the southern U.S. border a priority. In a day-one executive order, he once again declared a national emergency in response to what he said was an “invasion” of the United States by migrants. He also directed federal agencies to revive the “Remain in Mexico” policy—though a federal court blocked its reimplementation, and legal challenges are ongoing.
Under Defense Secretary Pete Hegseth, the military’s role in border security has expanded significantly. As part of his goal to achieve “100 percent operational control” of the southern border, the Pentagon has deployed armored Stryker combat vehicles—as well as thousands of National Guard and active-duty troops—and established several National Defense Areas along the boundary. These federally controlled zones enable the military to help enforce border security—a move that Senator Jack Reed (D-RI) says is a way to side-step the Posse Comitatus Act by “allowing military forces to act as de facto border police.”
The administration has also resumed construction of the U.S.-Mexico border wall and allocated roughly $170 billion in new funding for border and immigration enforcement via Trump’s “big, beautiful bill,” enacted in early July. This includes more than $46 billion for border wall construction and related projects.
Recommended Resources
These Backgrounders explain the U.S. asylum process and the U.S. refugee system.
This Congressional Research Service report explains CBP’s powers and limitations [PDF].
This report by the Migration Policy Institute’s Alan D. Bersin, Nate Bruggeman, and Ben Rohrbaugh examines the history of U.S. efforts to improve border security in the modern era.
Eric Schmitt, Helene Cooper, and Adriana Zehbrauskas of the New York Times explore how the military’s presence at the southern U.S. border has increased under the second Trump administration.
For The President’s Inbox podcast, CFR expert Edward Alden discusses Trump’s immigration policies and their likely effects on the U.S. economy.
Diana Roy is Senior Writer/Editor, Latin America, at CFR. Joseph Wehmeyer, Antonio Barreras Lozano, Zachary Laub, and Avery Reyna contributed to this report. Michael Bricknell and Will Merrow created the graphics. This article is published courtesy of the Council on Foreign Relations (CFR).
