How the U.S. Patrols Its Borders

Safeguarding the southern U.S. border, primarily the responsibility of the Department of Homeland Security (DHS), remains a contentious issue as an increasing number of migrants from Asia, Central America, and elsewhere seek to enter the United States via Mexico. President Donald Trump and Democratic lawmakers battled over funding for a U.S.-Mexico border wall, leading to a government shutdown, a presidential emergency declaration, and the deployment of thousands of National Guard and active-duty troops to the border.

Early in his administration, President Joe Biden reversed many Trump-era decisions, including halting construction of the border wall, easing some restrictions on asylum seekers, and pursuing regional diplomacy to target the root causes of irregular migration. However, a continuous flow of migrants arriving at the border is overwhelming the U.S. immigration system and testing federal enforcement capabilities.

What’s happening at the southern U.S. border?

Over the past decade, the U.S. immigration system has come under increasing strain. After arrivals plunged in 2020 due to the COVID-19 pandemic, illegal border crossings subsequently soared to record highs. In fiscal year 2022 (FY2022), U.S. immigration authorities apprehended more than 2.3 million people at the U.S.-Mexico border, the highest number ever recorded. As of mid-2023, that number had already surpassed 1.6 million, with about a quarter of all migrants coming from the so-called Northern Triangle countries of El Salvador, Guatemala, and Honduras. Other major countries of origin include Cuba, Haiti, Nicaragua, and Venezuela. Since May 2023, however, illegal crossings have unexpectedly dropped after the Biden administration lifted Title 42, an emergency public health order invoked by Trump that allowed border authorities to swiftly expel migrants.

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 highest concern for Border Patrol agents. Of the nine border sectors spread across Arizona, California, New Mexico, and Texas, the greatest number of apprehensions since 2020 have occurred along the easternmost part of the border, called the Rio Grande Valley sector, in Texas. This is followed by Del Rio, in southwestern Texas, which previously experienced only modest levels of migration.

The types of people 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 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 the 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 work [PDF]. Of CBP’s more than sixty thousand employees, nearly 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 by U.S. presidents several times over the past two decades to assist border agents with unauthorized immigration and drug trafficking. The George W. Bush administration deployed roughly 6,000 National Guard troops to the border, and the Barack Obama administration sent about 1,200 before trimming down the force. National Guard soldiers can be called to action by either a state governor or, in some cases, the president.

In 2018, National Guard personnel were deployed 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 to provide aerial and mechanical support. By August 2020, the Pentagon had sent more than 2,500 National Guard members [PDF] to aid CBP in the border states of Arizona, California, New Mexico, and Texas. The 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. 

Meanwhile, Texas Governor Greg Abbott continues to implement a $4 billion border enforcement program, known as Operation Lone Star, which aims to curb illegal border crossings with the help of the Texas National Guard and state troopers. Part of the plan includes deploying floating barriers in the Rio Grande to dissuade migrants from crossing, a move that the Department of Justice says lacks federal approval. Launched in 2021, Operation Lone Star has so far resulted in more than 371,000 apprehensions and over 27,000 criminal arrests. The program has received support from more than a dozen Republican governors, most notably Florida’s Ron DeSantis, who deployed more than one thousand National Guard troops and law enforcement officers to the Texas border.

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 as the country battled COVID-19. Approximately four thousand troops, a mix of active-duty and National Guard personnel, are currently stationed at the border. This includes 1,500 active-duty troops temporarily deployed to the border by the Biden administration ahead of the May 2023 expiration of Title 42, with the aim of relieving pressure on CBP agents.

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 [PDF]. Unlike the National Guard and CBP, 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 a 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 FY2022, more than six hundred officers and agents were attacked while on duty at the southern border, and over three hundred such incidents have already occurred in the first eight months of FY2023. The annual number of incidents involving use of force by CBP personnel rose steadily between 2017 and 2021, though it has since dropped; 275 people—both citizens and noncitizens—have been reported killed in confrontations with border agents since 2010.

Is there a threat to U.S. national security?

The Trump administration repeatedly framed border enforcement as a national security priority. It warned of Central American gang members and would-be South Asian and Middle Eastern terrorists joining caravans of migrants to infiltrate the United States. Then DHS Secretary Kirstjen Nielsen said in 2019 that CBP had blocked more than three thousand people with terrorist-like travel patterns or connections to terrorism from entering the United States at the southern border. However, the Washington Post challenged these figures.

In 2019, Trump declared a national emergency—which he extended in 2020—that allowed him to redirect some $10 billion from the military budget to fund construction of a wall along the U.S.-Mexico border, which he argued would keep out criminals and halt the flow of illegal drugs. The move triggered dozens of legal challenges, but the Supreme Court allowed the administration to begin using diverted funds pending litigation. However, while DHS later reported steep drops in apprehensions in areas where border wall sections were added, experts say migrants have long bypassed such barriers. As for drugs, CBP data shows that more are seized at legal ports of entry than between them.

How did Trump change the asylum process?

As a signatory to the 1951 UN Refugee Convention and its 1967 Protocol, the United States has committed to providing refuge to people with a “well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group or political opinion.” Applicants must demonstrate a credible fear that they would be killed or tortured if returned to their home country. The U.S. Immigration and Nationality Act makes asylum available to those already living in the United States as well as those who arrive at an official port of entry along its borders.

However, Trump denigrated the legal right to seek asylum as a “loophole” in immigration policy, subject to fraud and abuse. His administration pursued policies to block or otherwise discourage asylum seekers.

Zero tolerance. With long waits at ports of entry driving more and more asylum seekers to cross at unauthorized points, the Trump administration implemented a blanket zero-tolerance policy intended to increase prosecutions of undocumented migrants who crossed the border illegally, including asylum seekers. (Such prosecutions were relatively infrequent [PDF] under Presidents Bush and Obama, but reached a record level of more than 100,000 in 2019.) The government 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 apply for asylum in Mexico, but they frequently face unsafe conditions, and the country’s bureaucracy is already stretched beyond its capacity to process claims.

Pandemic restrictions. Claiming asylum became even more difficult during the pandemic after the Trump administration invoked Title 42; between 2020 and 2021, the policy was used to deport more than 1.2 million migrants. The approval rate for asylum seekers is about 30 percent, and research has shown that the chances of winning asylum increase fivefold when migrants have legal representation, which is not often the case. A growing backlog in the immigration courts—nearly 2.1 million cases as of January 2023—has meant long waits for claims to be heard; still, most asylum seekers appear for their court hearings. Many who lose their cases become undocumented, though in some instances they can appeal the decision.

How is Biden responding?

Biden has dismantled several Trump-era policies while preserving others. He quickly suspended federal construction of the southern border wall, though some governors continue to expand it, and rescinded Trump’s national emergency declaration. His administration also officially ended the zero-tolerance policy and has sought to end the controversial “Remain in Mexico” program, but it remains in place amid ongoing legal difficulties. 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; since it was implemented, illegal border crossings have slowed. The policy, however, faces legal challenges. Meanwhile, the administration reopened temporary facilities to house the swelling number of unaccompanied child migrants, who continue to be allowed to stay in the United States. 

To incentivize legal migration, meanwhile, the administration has expanded legal pathways for migrants to apply for asylum. These have included creating humanitarian parole programs for migrants from certain countries, opening regional processing centers in Colombia and Guatemala, and building upon migration-related pledges made at the 2022 Summit of the Americas. Still, the administration continues to face criticism over reports that it could begin detaining families again, as well as its decision to fast-track asylum screenings at the border. At the same time, Biden’s own efforts to pass comprehensive immigration reform remain stalled in Congress, and some analysts say his administration’s four-year, $4 billion plan to address the drivers of Central American migration is not doing enough.