Foreign policy & defense
Posse Comitatus Act
The 1878 law barring use of the federal Army and Air Force for domestic law enforcement, unless Congress or the Constitution authorizes it.
In full
The Act limits federal military forces, not the National Guard under state command. The Insurrection Act is the main statutory exception, allowing the President to deploy federal troops to suppress insurrection or enforce federal law in defined circumstances. The line between military support and enforcement is the operative question in most disputes.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.