The US is sending thousands of troops to “harden” the border with Mexico, Pentagon officials said today (Oct.29), as thousands of asylum-seekers approach on foot from Central America.

The armed forces will send 5,200 personnel, three helicopters, several military planes, and more than 150 miles of razor wire to border locations, starting in Texas, Air Force Gen. Terrence J. O’Shaughnessy, commander of North American Aerospace Defense Command and US Northern Command, told reporters Monday (Oct. 29) afternoon, confirming earlier reports.

Customs and Border Protection officers will continue to enforce immigration law on the border, the general stressed, and the thousands of military personnel dispatched on “Operation Faithful Patriot” will be “complementary to that effort.” In addition, he said in response to question, “everything we are doing is in line with and adherence to posse comitatus.”

The Defense Department press conference marked the second time a Trump administration official was asked about “posse comitatus” today. Reporters also grilled White House press secretary Sarah Huckabee Sanders about whether Trump would suspend habeas corpus—the right for anyone who is arrested to appear before a judge—in order to stop asylum seekers. She did not rule it out, sparking concerns.

A posse comitatus is defined as a group summoned by the local sheriff to keep the peace or enforce an opposed legal precept, but in this case the term refers to an act first passed after the US Civil War that makes it illegal for the armed forces to be used to uphold domestic laws.

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.

At its essence, the law reflects the American historical preference for stronger state governments and local authorities over centralized federal oversight and control. It was first created to “end the use of federal troops to police state elections in former Confederate states,” as the Rand Corporation, the military think tank, explains.

Giving border control over to the military would be a controversial decision that could cost Donald Trump support even among the most far-right of voters and could damage the Republican party ahead of the Nov. 6 midterm elections. It would also signal that the Trump administration is willing to use the US military in unconventional ways.