The Supreme Court ruled on December 23, 2025, in Trump v. Illinois that President Donald Trump was extremely limited in his ability to federalize a state’s National Guard. The 6-3 decision interpreted a key provision of federal law governing when the president may call state National Guard units into active federal service.

The Court specifically interpreted 10 U.S.C. § 12406(3), which empowers the president to federalize members of a state’s National Guard if he is “unable with the regular forces to execute the laws of the United States.” The justices concluded that the term “regular forces” in this statute refers to the regular forces of the United States military. “We conclude that the term ‘regular forces’ in §12406(3) likely refers to the regular forces of the United States military. This interpretation means that to call the Guard into active federal service under §12406(3), the President must be ‘unable’ with the regular military ‘to execute the laws of the United States,’” the Court stated.

The Trump administration had sought to federalize the Illinois National Guard, asserting that local police were unable to adequately protect U.S. Immigration and Customs Enforcement (ICE) agents. The administration made similar claims regarding the National Guard in California and Oregon, arguing that federal law enforcement personnel required military-grade support due to insufficient local protection. The Court’s ruling effectively blocked such actions unless the president could first demonstrate an inability to enforce federal law using the regular U.S. military.

The decision situates §12406(3) within the broader legal framework restricting the domestic use of military forces. The Posse Comitatus Act, adopted in 1878, prohibits the United States military from being used for domestic law enforcement except in very limited circumstances. The law makes it a federal crime to use the military within the United States except as expressly authorized by the Constitution or a federal statute. By tying the activation of the National Guard under §12406(3) to the capabilities of the regular military, the Court reinforced the longstanding boundary between civilian policing and military involvement in domestic affairs.

Why It Matters

The ruling clarifies a rarely litigated provision of federal law and reaffirms constitutional limits on presidential power to deploy military-like forces domestically. It directly affects how future presidents may respond to civil unrest or federal enforcement challenges in states where local authorities are perceived as uncooperative or overwhelmed.

By requiring the president to first exhaust the capacity of the regular U.S. military before invoking §12406(3), the Court has set a high threshold that aligns with the Posse Comitatus Act’s purpose of preventing military intrusion into civilian law enforcement. The decision ensures that federalization of state National Guard units cannot serve as a workaround to domestic military deployment restrictions.