A confidential Department of Homeland Security memo issued shortly after President Donald Trump returned to office gave officials a legal rationale for using National Guard troops as immigration officers, opening the door to a constitutional battle over the use of military personnel for domestic law-enforcement purposes.
The practice is limited by a once-obscure 1878 law called the Posse Comitatus Act, which is meant to restrict the president from using troops within U.S. borders. But in an effort to carry out Trump’s aggressive mass-deportation push, federal officials at multiple junctures ran into legal challenges while citing the law — including in Los Angeles, where Trump deployed National Guard troops against Gov. Gavin Newsom’s wishes and was later rebuked in court for running afoul of Posse Comitatus.
The April 2025 memo, obtained by American Oversight through a Freedom of Information Act request and shared with NOTUS, argues that National Guard members can be designated as federal immigration officers when they are ordered to serve outside their home states. The document authorized them to “perform immigration functions,” including arresting unauthorized immigrants — though no such instances have been reported.