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.”