How the U.S. Patrols Its Borders

How the U.S. Patrols Its Borders

President Trump sent U.S. troops to the border with Mexico to supplement the work of authorities there. President Biden pledges a new approach as pressures mount along the boundary.
Unaccompanied children seeking U.S. asylum sit near a Border Patrol vehicle in Penitas, Texas.
Unaccompanied children seeking U.S. asylum sit near a Border Patrol vehicle in Penitas, Texas. Adrees Latif/Reuters
  • U.S. Customs and Border Protection (CBP), along with other agencies, regulates trade and travel at U.S. ports of entry and across the country’s borders.
  • President Trump deployed thousands of National Guard and active-duty military personnel to help secure the border with Mexico and declared a national emergency that unlocked funding for the border wall.
  • President Biden, facing a rise in migrants arriving at the southern border, has reversed Trump-era policies and seeks to fundamentally reform the immigration enforcement system.

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

More From Our Experts

Early in his administration, President Joe Biden has begun reversing Trump-era decisions, including halting construction of the wall, easing some restrictions on asylum seekers, and pursuing regional diplomacy to target the causes of migration. However, increasing arrivals at the border are once again testing U.S. enforcement capabilities.

What’s happening at the southern border?

More on:

United States

Border and Port Security


Immigration and Migration

Refugees and Displaced Persons

In early 2021, the Biden administration said it was on track to face the largest number of migrants along the southern border in two decades, after arrivals plunged in 2020 amid the coronavirus pandemic. President Biden has continued pandemic-related ejections of most migrants, but unaccompanied children and some families have been allowed to remain in the United States. In an echo of upswings in migration in fiscal years 2014 and 2019, the immigration system has come under increasing strain as thousands of unaccompanied children are now in U.S. custody, many of whom are held for days in border facilities that are not equipped for them. Apprehensions along the southern border are spiking, with the total in March 2021 resembling the monthly peaks of the early 2000s.

Where are most border crossings?

The southern 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. In recent years, the greatest number of apprehensions have occurred along the easternmost part of the border, called the Rio Grande Valley sector, in Texas. However, the Trump administration built less new border barrier in that sector than any other, partly because it had difficulty procuring land from private owners. 

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. Migration from Mexico fell sharply beginning in the mid-2000s, while that from other regions has risen. Over the past decade, many have been asylum-seeking minors and families from Central America’s Northern Triangle: El Salvador, Guatemala, and Honduras. They have fled gang recruitment, protection rackets, abusive police, inept criminal justice systems, and poverty. In 2020, the pandemic and several destructive hurricanes further deteriorated conditions in the Northern Triangle, driving more migration north. 

More From Our Experts

Who is responsible for U.S. border security?

Securing the borders primarily falls to U.S. Customs and Border Protection (CBP), a branch of the Department of Homeland Security. Alongside agencies such as the Transportation Security Administration and Drug Enforcement Administration (DEA), CBP is responsible for overseeing 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 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 in recent years 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.

More on:

United States

Border and Port Security


Immigration and Migration

Refugees and Displaced Persons

In April 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 up to about 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. The administration also extended their border mission through September 2021. Shortly after taking office, the Biden administration said that about 3,600 military personnel will continue assisting CBP and DHS until then, though it is unclear whether National Guard troops are included in this total. 

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 nearly six 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 President Donald Trump made immigration a core issue. 

Since then, the number of active-duty troops assigned to the southern border has varied. It notably rose in early 2019 after the Pentagon deployed thousands more troops. 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.

What are the rules of engagement?

The 1878 Posse Comitatus Act limits the U.S. military’s role in enforcing domestic laws, meaning that interactions between active-duty troops and migrants at the Mexican border is restricted. They can neither detain and deport unauthorized immigrants nor conduct searches and seizures. Like the National Guard soldiers, they largely provide indirect support, such as conducting aerial surveillance and repairing or reinforcing infrastructure. 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 to affect an arrest,” 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.

Border Patrol
Border Patrol agents apprehend migrants at the southwestern border. John Moore/Getty

CBP personnel face hundreds of assaults each year. In fiscal year 2020, more than nine hundred  officers and agents were attacked in over four hundred incidents. The annual number of incidents involving use of force by CBP personnel has risen steadily since at least 2017, and dozens of people—both citizens and noncitizens—have been reported killed in confrontations with border agents in recent years.

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. 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. The Washington Post challenged these figures, and some state governors recalled their National Guard personnel due to skepticism that the situation there constituted a national security crisis.

Trump also argued that a wall along the southern border, in addition to other security measures, would keep out criminals and halt the flow of illegal drugs. He called on Congress to approve more than $5 billion to fund the construction of more than two hundred miles of wall, a demand that was rejected by Democratic lawmakers and led to a month-long partial government shutdown. 

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 wall construction. The move triggered dozens of legal challenges, but the Supreme Court allowed the administration to begin using diverted funds pending litigation. 

The efficacy of these measures is unclear. Trump’s DHS reported steep drops in apprehensions in areas where border wall sections were added, but experts say migrants have long bypassed such barriers. Additionally, factors including the pandemic could make it difficult to determine the wall’s effectiveness for years to come. 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 1967 protocol to the UN refugee convention, 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 in the United States as well as those arriving at 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, Trump’s administration implemented a blanket “zero-tolerance” policy by which it sought to criminally prosecute all adults who crossed the border illegally, including asylum seekers. (Such prosecutions were relatively infrequent [PDF] under Presidents Bush and Obama.) Under the policy, the government separated children from their parents and detained minors; though Trump signed an executive order in 2018 to end separations, they continued. In the 2019 fiscal year, his administration prosecuted a record number of people for immigration-related offenses, including more than twenty-five thousand for felony illegal reentry and nearly eighty-one thousand for misdemeanor improper entry. 

Narrower criteria. In 2018, Attorney General Jeff Sessions narrowed the criteria for asylum [PDF], instructing judges that gang or domestic violence generally does not qualify as grounds for asylum. Trump signed an executive order later that year denying migrants the right to seek asylum outside ports of entry. However, federal courts blocked both of these moves.

Migrant Protection Protocols. In 2019, a new policy called the Migrant Protection Protocols (MPP) began requiring migrants who were waiting for their asylum claims to be reviewed to stay in Mexico, with the Mexican government agreeing to offer them visas and work permits. Some migrants applied for asylum in Mexico, though they frequently face unsafe conditions there and the country’s bureaucracy is already stretched beyond its capacity to process claims. 

Pandemic restrictions. Claiming asylum became even more difficult during the COVID-19 pandemic after the administration issued a public health order that allows border authorities to swiftly expel migrants, including asylum seekers.

Success rates for asylum seekers vary widely by court, but are overall quite low, often less than 10 percent. A growing backlog in the immigration courts—1.26 million cases at the end of fiscal year 2020—has meant long waits for claims to be heard; still, most asylum seekers appear for their hearings. Many who lose their cases join the ranks of the undocumented population.

How is Biden responding?

Biden has begun dismantling Trump’s policies. He quickly paused construction of the southern border wall and rescinded Trump’s national emergency declaration. His administration also officially ended the zero-tolerance policy, stopped sending migrants back to Mexico under MPP, and started allowing those returned to Mexico with pending asylum cases to enter the United States.

Yet, the administration has continued expelling most migrants under the pandemic-related public health order. Meanwhile, it reopened temporary facilities to house the swelling number of unaccompanied child migrants, who are allowed to stay in the United States; dispatched officials from the Federal Emergency Management Agency to help process them; and ramped up their releases to sponsors.

Biden has pledged to comprehensively reform the U.S. immigration system. He sent to Congress an immigration bill that would expand border surveillance, increase oversight of border authorities, and create new standards for CBP’s treatment of migrants. His plans also focus on helping Central American countries address the root causes of migration: the proposed legislation would provide $4 billion in new aid for this regional strategy. In late March, Biden named Vice President Kamala Harris to lead the U.S. response.

Recommended Resources

CFR’s Paul J. Angelo analyzes why so many Central Americans are arriving at the U.S. southern border and how the Biden administration is responding.

The Congressional Research Service explains CBP’s powers and limitations [PDF].

After traveling the U.S. southern border, USA Today offers a deep dive into life along the boundary and the implications of the wall.    

This CFR Backgrounder looks at the detention of child migrants in the United States.

This CFR Info Guide lays out the shrinking options for refugees.

Creative Commons
Creative Commons: Some rights reserved.
This work is licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) License.
View License Detail

Avery Reyna contributed to this report.

For media inquiries on this topic, please reach out to [email protected].

Top Stories on CFR


The Chinese government’s increasingly aggressive diplomacy has hurt its image in much of the world. It’s unlikely to improve anytime soon.

United States

Richard Haass, president of the Council on Foreign Relations, sits down with James M. Lindsay to assess how the Biden administration has handled foreign policy in its first year in office.

NATO (North Atlantic Treaty Organization)

Russian leaders have watched with mounting resentment as the transatlantic alliance has nearly doubled its membership since the end of the Cold War. President Vladimir Putin has drawn a red line in Ukraine.