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.
The memo, penned by then-DHS general counsel Joseph Mazzara, argues that National Guard members can take on a different legal status when they are assigned to another state, effectively making them federal employees.
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Politico first reported the memo.
Trump has repeatedly threatened to invoke the Insurrection Act, which would give him broader authority to deploy troops domestically. The law was originally written to provide the president greater options to respond to severe unrest — though critics argue the existence of undocumented immigrants does not qualify.
During Trump’s second term, the Supreme Court has limited his administration’s ability to deploy the National Guard for immigration enforcement, ruling in December that local forces would need to be incapacitated in some way before military personnel could be used for domestic purposes. The decision came after Illinois filed a legal challenge following a draconian immigration crackdown known as “Operation Midway Blitz” that employed National Guard troops.
National Guard troops have still assisted with background immigration work and provided security for federal agents carrying out immigration arrests, but the Trump administration has not yet openly directed the service members to directly carry out arrests.
Chioma Chukwu, the executive director of American Oversight, a nonprofit government watchdog group, said the memo should be alarming to Americans everywhere.
“Combined with the administration’s escalating claims about noncitizen voting, this theory could be used to put military forces near polling places under the guise of immigration enforcement,” Chukwu said in a statement. “The stakes extend well beyond immigration: This memo gives the president a ready-made workaround for using military power at home whenever the administration decides its agenda demands it.”
DHS did not immediately respond to a request for comment from NOTUS, but told Politico that the letter would “speak for itself.”
Homeland Security Secretary Markwayne Mullin has separately sought to use DHS to investigate alleged voter fraud. He and other administration officials have suggested that noncitizens are voting in large numbers and potentially swaying the results of U.S. elections — though they have provided little evidence to back up their claims.
“We will scrub all election records, looking for illegal aliens and those who are ineligible to vote, including those that somehow voted, yet they were deceased,” Mullin said in July.
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