Reconstruction-era law limits National Guard as border guards

In recent days, President Donald Trump has threatened to deploy National Guard members to block a mass-immigration situation at the Mexican border. However, an 1878 law may effectively limit the Guard’s ability to act there directly.

The Posse Comitatus Act dates back to the Rutherford B. Hayes era in Washington after federal troops left formerly rebellious states as part of the pact that ended the Reconstruction era. The act was originally intended to make it difficult for federal forces to execute criminal laws in those southern states, as they had between 1865 and 1878.

The act, most recently amended in 1994, says that “Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.”

The law is understood to infer that any armed forces from the federal government, including National Guard units mobilized for federal duty, may not be able to enforce immigration laws at the national border directly.

The Congressional Research Service has analyzed this topic at length since the George W. Bush era.

“The use of the military to enforce immigration or criminal laws at the border could run afoul of the Posse Comitatus Act, unless an exception applies,” the CRS said in April 2018.

“The Constitution empowers Congress to authorize the militia to be called forth to execute federal law. Congress has used this power to authorize the President to use the regular Armed Forces and the National Guard in cases of insurrection against state governments, obstruction of federal laws, or to protect civil rights,” the CRS explains. “The armed forces do not appear to have a direct legislative mandate to protect or patrol the border or to engage in immigration enforcement.”

The National Guard or regular federal forces could assist the United States Border Patrol by offering technical assistance and training, and intelligence, logistical and administrative duties. Furthermore, the CRS says a federal statute excludes “direct participation by a member of the Army, Navy, Air Force, or Marine Corps in a search, seizure, arrest, or other similar activity unless participation in such activity by such member is otherwise authorized by law.”

Another law, Section 502 of Title 32, U.S. Code, does permit the use of the National Guard if it takes part in "homeland defense activity" that is “undertaken for the military protection of the territory or domestic population of the United States, or of infrastructure or other assets of the United States determined by the Secretary of Defense as being critical to national security, from a threat or aggression against the United States.”

But under Title 32, national guard troops remain under the control of each state’s governor, and when Secretary of Defense James Mattis requested 4,000 guard troops in April for border operations, the Defense Department said the deployments needed each governor’s approval.

“The troops will not perform law enforcement activities or interact with migrants or other individuals detained by DHS without approval from Mattis,” the department memo said.” The National Guard’s efforts will include aviation, engineering, surveillance, communications, vehicle maintenance, and logistical support.”

But what happens if someone violates the Posse Comitatus Act? The CRS looked at this scenario back in 2012 in a more detailed analysis. With rare exceptions, the act has been complied with since 1878, it said.

There had never been a reported prosecution of a criminal violation of the act, as of 2012. But the act has “been invoked with varying degrees of success to challenge the jurisdiction of the courts; as a defense in criminal prosecutions for other offenses; as a ground for the suppression of evidence,” among other civil court uses.