Posse Comitatus: The 140-Year-Old Law That Affects U.S. Troops at the Border

U.S. troop at Mexican border
A U.S. Army soldier is seen from the Mexican side fortifying the U.S.-Mexico border wall with barbed wire on Nov. 26, 2018, in Mexicali, Mexico. Luis Boza/VIEWpress/Corbis via Getty Images

Whether you believe everyone should have a right to life, liberty and the pursuit of happiness, American-style, or whether you believe Americans should guard their sovereignty with everything they've got — or, perhaps, you believe both; those views may not be mutually exclusive — we all can agree on this: The U.S.-Mexico border remains a flashpoint in American politics.

There are troops there. A president vowing to close the border and defend the country with "lethal force" if necessary. A surge of immigrants wanting in. Tear gas, concertina wire, walls ... .


This, too, is undeniable: The problem is not going away any time soon.

The Immigrant Question

A group of immigrants from Central America trying to get into the U.S. — some legally, some it is supposed by whatever means they can — prompted President Donald Trump in November 2018 to send more than 5,000 troops to the Mexican border. That, by the way, is more troops than the U.S. has in war-ravaged Syria.

The presidential order immediately raised a slew of questions, like: Why so many troops for a few thousand unarmed immigrants? What are these troops doing down there? What can they do? Can U.S. troops fire on foreign civilians? Can they use deadly force? Should they?


Amid all the uncertainty, one question nagged at many observers: Is sending federal troops to the border even legal? As it turns out, many suggest, because of a portion of a 140-year-old U.S. law known colloquially as the Posse Comitatus Act, that it isn't.

The Congressional Research Service explains:

Americans have a tradition, born in England and developed in the early years of our nation, that abhors military involvement in civilian affairs, at least under ordinary circumstances. It finds its most tangible expression in the 19th century Posse Comitatus Act ... which forbids use of the Army and (as amended) the Air Force to execute civil law except where expressly authorized.

Posse comitatus — Latin for "power of the county," an idea that leaves law enforcement to local authorities, not the feds — only can be excepted, the statute says, "in cases and under circumstances expressly authorized by the Constitution or Act of Congress."

In this instance, Trump has declared a constitutional exception, under Article IV, Section IV, which guarantees that the federal government will protect states from "invasion" and "domestic violence."

Whether that's valid depends on which side you stand.

"The lawyers would call it a gray area," says William C. Banks, a law professor emeritus at Syracuse University and the co-author of "Soldiers on the Home Front: The Domestic Role of the American Military." "Posse comitatus isn't the only legal problem. Everything that the United States does has to be based on some legal authority. It's not clear to me what the legal authority is to even send the military down there. [Trump] hasn't said, and no one else has said, other than 'We need them down because we got some bad people coming at us on the border.' That's not a legal theory.

"In my view, the strongest argument for him to have the military at the border is a physical threat to the United States, which would trigger his Constitutional powers as Commander in Chief. I don't know, from what I've seen, that the facts justify that. Without that, he has no authority to be there."

Central American migrant caravan
Hundreds of members of the Central American migrant caravan move in the early morning hours toward their next destination of Puebla on Nov. 5, 2018, in Cordoba, Mexico.
Spencer Platt/Getty Images


Invasion or Not?

Bypassing the Posse Comitatus Act gets tricky, as the Congressional Research Service points out. If the law is adhered to strictly — if the feds keep hands off of local law enforcement — an uprising can happen (as in the 1786-87 Shays Rebellion). If, as with Trump's troops, the feds jump into an already volatile situation, they could exacerbate it, and a Kent State-type tragedy could occur (though that involved the Ohio National Guard, not federal troops).

Many, including Banks, believe that Trump sent the troops to the border as a political ploy, a way of stoking his Republican electoral base before the midterm elections. But even if that isn't the case, even if the president truly believed an "invasion" of immigrants was imminent and that it was a threat to the U.S., 5,000-plus troops seems, at least so far, a bit much.


States already had deployed National Guard troops to the border when the administration sent in the federal troops. The U.S. Customs and Border Protection (CBP) is there. They are all coordinating with Mexican authorities. Despite a skirmish on Nov. 25, 2018, that involved tear gas (released by the CBP, not the feds) and rock-throwing immigrants trying to breach the border, it doesn't appear that the U.S. military is needed. So far, federal troops have been relegated to providing support to the CBP (transportation, stringing concertina wire, building barriers). Most U.S. troops aren't even carrying weapons.

"The [administration's] order allows them to potentially do some things that would cause a lot of us to get nervous," Banks says. "But [U.S. Secretary of Defense James] Mattis said the other day, 'Look, they aren't carrying their weapons right now. I don't think they'll have to.'"

If that time ever comes, then thousands of troops may be called for. Until then, though, local law enforcement seems to have this handled.

"If there's a real threat to the border, yes, [the president] has constitutional authority to act. And then he can sort of unleash the troops," Banks says. "It's just hard to imagine that happening."