Trump's domestic troop deployment tests the limits of a nearly
150-year-old law
[August 13, 2025]
By SAFIYAH RIDDLE
As President Donald Trump pushes the bounds of military activity on
domestic soil, a debate has emerged over a nearly 150-year old law that
regulates when federal troops can intervene in state issues.
About 800 National Guard troops filed into Washington, D.C., on Tuesday
after President Donald Trump said — without substantiation — that they
were needed to reduce crime in the “lawless” national capital. Thousands
of miles away, a judge in California is hearing arguments about whether
the president's recent decision to federalize Guard personnel in Los
Angeles during protests against immigration raids violated federal law.
Trump has also created militarized zones along the U.S.-Mexico border as
part of a major shift that has thrust the army into immigration
enforcement like never before.
The cases in both California and Washington mainly hinge on Posse
Comitatus Act, which passed in 1878 and largely prevents the military
from enforcing domestic laws. Experts say that in both cases there are
clear limitations to the law's enforcement.
Here is what to know about the law.
Posse Comitatus Act stops military from enforcing US law
The Posse Comitatus Act is a criminal statute that prevents the military
from enforcing domestic law. It also prevents the military from
investigating local crimes, overriding local law enforcement or
compelling certain behavior.
Posse Comitatus can be bypassed by a congressional vote or in order to
defend the Constitution. The Insurrection Act of 1807 can also trigger
the suspension of the Posse Comitatus Act and allows the president to
deploy the military domestically in cases of invasion or rebellion.
There is an exception for the U.S. Coast Guard, which has some law
enforcement responsibility. The military is also allowed to share
intelligence and certain resources if there is an overlap with civilian
law enforcement jurisdiction, according to the Library of Congress.
Law was enacted after Reconstruction era
The law was enacted in 1878 following the post-Civil War era known as
Reconstruction. Pro-segregationist representatives in Congress wanted to
keep the military from blocking the enforcement of Jim Crow laws that
allowed racial segregation.
But the spirit of the law also has roots going all the way back to the
Revolutionary War, when the founders of the United States were scarred
by the British monarchy's absolute military control, said William C.
Banks, a professor at the Syracuse University College of Law.
“We have a tradition in the United States, which is more a norm than a
law, that we want law enforcement to be conducted by civilians, not the
military,” Banks said.
That ethos — ingrained in National Guard personnel starting in basic
training — becomes especially powerful in the case of the Posse
Comitatus Act, because the law has hardly been tested before now, said
Steve Vladeck, a professor of law at the Georgetown University Law
Center.
“There is no authoritative precedent on exactly where these lines are,
and so that’s why over the years the military’s own interpretation has
been so important,” Vladeck said.
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Federal agents stage at MacArthur Park, July 7, 2025, in Los
Angeles. (AP Photo/Damian Dovarganes, File)
Law applies to ‘federalized’ troops
The Posse Comitatus Act typically doesn't apply to the National Guard
because members of the Guard report to the governor, not the federal
government.
But when Guard personnel are “federalized" they are bound by the act
until they are returned to state control, according to the Brennan
Center for Justice.
The state of California said in a federal lawsuit that the Trump
administration violated the act when it deployed National Guard soldiers
and U.S. Marines to Los Angeles following June protests over immigration
raids.
The Trump administration has argued that the Posse Comitatus Act does
not apply because the president used a provision known as Title 10 to
federalize the troops. It allows the president to call the National
Guard into federal service when the country “is invaded,” when “there is
a rebellion or danger of a rebellion against the authority of the
Government,” or when the president is otherwise unable “to execute the
laws of the United States.” Attorneys for the federal government also
argue that the troops are not enforcing domestic laws and are only
acting to protect federal property and agents.
In Washington, by contrast, the president is already in charge of the
National Guard and can legally deploy troops for 30 days without
congressional approval.
Vladeck said that both deployments over the past three months suggest
that the Trump administration “appears to be trying to dance around the
Posse Comitatus Act" rather than disregard it altogether.
"There is a lot in the water about the Trump administration being
lawless. What is striking is actually how much the administration is
trying to wrap itself in the law,” Vladeck said.
Law depends on executive branch policing itself
Beyond the legal exceptions written into the law, there is a practical
question of how to enforce it, said Joseph Nunn, counsel in the Brennan
Center for Justice’s Liberty and National Security Program.
Because the Posse Comitatus Act is a criminal statute, not a civil one,
the U.S. Department of Justice is responsible for prosecution in
criminal court, Nunn said.
“It's premised on the executive branch policing itself,” he said. That
leaves unclear legal standing for whether a state government like
California's has a right to sue in civil court in the first place.
The ruling in the California case will likely be a narrow interpretation
based on the circumstances of the Guard's deployment in Los Angeles,
Vladeck said. But he said it could still dictate how the administration
uses the Guard in other cities like Chicago and New York, where Trump
has threatened to federalize troops next.
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