On Sunday, Secretary of Defense Pete Hegseth and Secretary of Homeland Security Kristi Noem are expected to deliver a report to President Trump on the southern border. The report, mandated in an executive order Trump signed on Inauguration Day, is expected to include recommended actions for the country to “obtain complete operational control”—including whether the president should take the extraordinary step of invoking the Insurrection Act.

Since Trump took office, the administration has increased the military’s presence at the southern border, deploying about 10,000 troops there, along with combat vehicles and equipment. Last week, it transferred a strip of federal land along the border from the Interior Department to the Defense Department, which means that non-military personnel who enter it can be detained.

But invoking the Insurrection Act would escalate the militarization of the border—and potentially elsewhere in the country. In short, it would give the president very wide latitude to use the military on U.S. soil, free from the major checks and balances that usually come with deploying the military domestically.

We’ve written about the Insurrection Act and domestic deployment of the military before. So as a refresher, here’s our rundown of what it is and what it all means. You have questions? We have answers.

Q. So what’s this executive order about?

A. Just after he took office, Trump declared a national emergency at the southern border.

“America’s sovereignty is under attack. Our southern border is overrun by cartels, criminal gangs, known terrorists, human traffickers, smugglers, unvetted military-age males from foreign adversaries, and illicit narcotics that harm Americans, including America,” he wrote. “This assault on the American people and the integrity of America’s sovereign borders represents a grave threat to our Nation.” Critics and many policy analysts say the president is overstating the security threat.

Among other things, the order directed the military to move troops and assets to the border and to work with Homeland Security to build physical barriers along the border. It also directed Hegseth and Noem to jointly submit a report 90 days later—that’s this Sunday—recommending any further actions needed to “control” the border. The executive order specifically directed them to include a recommendation on whether Trump should invoke the Insurrection Act.

Q. What’s the Insurrection Act?

A. The Insurrection Act is a series of laws dating back to 1792. They grant the president essentially unchecked authority to use the military “whenever the president considers that unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States make it impracticable to enforce the laws of the United States.”

The act has been used fairly regularly throughout American history—but not recently. In fact, we are currently in the longest period in which it hasn’t been used.

Q. When has the Insurrection Act been used before?

A. Presidents have invoked the Act a total of 30 times in our country’s history. The first time was in 1794, when George Washington put down the Whiskey Rebellion, an uprising in western Pennsylvania by farmers who opposed a new tax on whiskey production.

But the majority of times the Insurrection Act has been invoked have been in defense of civil liberties—notably to suppress white supremacists in the Reconstruction-era south, as well during the Civil Rights struggle in the 20th century.

The last invocation of the Insurrection Act was 1992, when President George H.W. Bush sent active duty Marines and National Guardsmen into L.A. during the riots after the police beating of Rodney King.

Q. What’s the difference between active duty and the National Guard?

A. The National Guard is a unique force. Most of the time, it’s under the command of a state governor, who can use Guardsmen for any number of missions in their state—responding to natural disasters, filling in as healthcare workers during a pandemic, patrolling the subway over crime concerns. State governors can also lend their National Guard units to other states that request them. For instance, more than a dozen states have sent Guard units to the southern border at Texas Gov. Greg Abbott’s request.

But the president can also federalize National Guard troops, or bring them under presidential control. In that case, they’re considered the same as active duty military troops and are subject to certain additional safeguards to ensure we don’t use the military as a domestic police force.

Q. What are those safeguards?

A. The big one is called the Posse Comitatus Act, a law dating back to the Reconstruction era after the Civil War. It prevents the military from acting in a law enforcement capacity. That means that under normal circumstances, the military—including National Guard troops under presidential control—can’t start conducting searches of civilians or arresting protesters.

Q. So what does the Insurrection Act mean?

A. When a president invokes the Insurrection Act, he can send in any troops, including active duty military or federalized National Guard units. And when troops are mobilized under the Insurrection Act, posse comitatus does not apply: Although they are under federal command, they may act in a law enforcement capacity.

While the Insurrection Act has been used to safeguard civil rights—including, famously, to ensure school desegregation in the South—it has no provisions for oversight or checks and balances: The president alone may decide when federal troops are needed to “enforce the laws of the United States,” and he can federalize National Guard units without their governor’s consent.

Q. So can troops do basically anything under the Insurrection Act?

A. No. Invoking the Insurrection Act does not suspend the Constitution or other civil liberty protections. Military troops deployed under the Insurrection Act still have to respect things like the 4th and 5th Amendments—no unreasonable searches and seizures, no taking life, liberty, or property without due process of law—just as law enforcement officers do.

But the Insurrection Act effectively leaves it up to the president to unilaterally determine what might constitute a rebellion, and to decide when to invoke his or her authority to call up the military to address it.

This War Horse story was reported by Sonner Kehrt and edited by Mike Frankel. Hrisanthi Pickett wrote the headlines.