With a so-called “caravan” of several thousand individuals trekking through Mexico and bound for the U.S., President Donald Trump is considering the option of sending U.S. troops to help close the southern border.

Here are the basics of how the military can and can’t be used at the border.

What laws govern the use of the military in the U.S.?

The Posse Comitatus Act prohibits the Army and Air Force from being used to enforce the laws of the U.S. with only select exceptions. Additional statues and Department of Defense policy directives have further applied Posse Comitatus to the Navy and Marine Corps. The only branch of the military with broad law enforcement authority is the Coast Guard.

Other specific exceptions where the military can enforce laws include:

• Emergency authority in the event of “sudden and unexpected civil disturbances, disasters, and calamities”
• Protection of federal property and functions
• Insurrection or rebellion
• When the execution of state or federal laws are obstructed or prevented, such that the rights of citizens are denied or the course of justice is impeded
• The National Guard when acting under authority of the state governor.

Under what authority can troops be deployed to the border?

The National Guard can carry out law enforcement missions when it remains under the control of the governor, avoiding the restrictions of Posse Comitatus. This can happen in two ways:

• State Active Duty: The National Guard are called up by, and remain under the control of the governor. They are deployed at the expense of the state.
• Title 32: Similar to State Active Duty, Title 32 of U.S. law allows a governor to call up and control that state’s National Guard, but the guardsmen are paid by the federal government.

The Trump, Obama, and George W. Bush administrations all deployed National Guard to the border under these provisions. Even though they are allowed to carry out law enforcement activities, as a matter of policy, these deployments have always been in a supporting role, such as aiding in logistics and reconnaissance, rather than front-line apprehension duty.

If the National Guard is called up under the control of the federal government, known as Title 10, then they generally cannot carry out law enforcement duties.

Will deploying the military stop these caravans?

The U.S. needs strong border security, and there are many active measures the U.S. can take to ensure that individuals cannot sneak across our border. These include barriers, technology, and personnel deployed in ways that maximize efficiency.

But unfortunately, the U.S.’s problem right now isn’t finding and stopping those who cross the border illegally. The U.S. has actually gotten quite good at that.

The real problem with the U.S. immigration system is a weak asylum system, as well as loopholes that prevent proper enforcement of U.S. laws. Because of these loopholes, if an individual comes across the border illegally with a child, they are generally released into the U.S., as current law and court decisions require.

If more troops are deployed to the border, they might help improve the number of border-crossers apprehended, but the Department of Homeland Security would still be releasing many of them.

Border security is only as good as the enforcement that backs it up. Only when U.S. laws are well enforced will they deter more illegal immigration.