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As Justice Robert H. Jackson would write in his celebrated concurring opinion in the Steel Seizure case, during the Truman administration, the Constitution’s limitation on domestic use of the military to three sets of emergencies, “written at a time when the militia rather than a standing army was contemplated as the military weapon of the Republic, underscores the Constitution’s policy that Congress, not the Executive, should control utilization of the war power as an instrument of domestic policy.”

At first, Congress was both thoughtful and careful in how it delegated such crisis power to the president. A 1792 statute required a federal judge to sign off on the president’s determination that the militia needed to be called forth, and also imposed strict time limits on how long the president could use the militia (and which states’ militias he could use for different purposes). President George Washington hewed closely to those mandates in suppressing the 1794 Whiskey Rebellion—the first true domestic security crisis the country faced under the Constitution.

But Congress took the wrong lessons away from this early experience. In 1795, it removed some of the most important checks on the use of the militia for domestic emergencies. And far more significantly, in 1807, a one-sentence statute enacted on the very last day of the Ninth Congress (and signed into law, more than a little ironically, by President Thomas Jefferson) provided that the president could use federal regulars—the standing army of which the Founders had feared—in most of the circumstances in which resort to the militia had already been authorized.

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Numerous statutory tweaks followed, but the structural features remained the same. Under the Insurrection Act as it stands today:

Whenever the President considers that unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings, he may call into Federal service such of the militia of any State, and use such of the armed forces, as he considers necessary to enforce those laws or to suppress the rebellion.

In other words, if the president determines that ordinary law enforcement is inadequate to enforce federal law, he can deploy the military to assist. And although Congress in the Posse Comitatus Act of 1878 generally prohibited use of the federal military for domestic law enforcement, the Insurrection Act was always understood as the principal exception to that general rule.

As documented in a comprehensive three-volume history by the U.S. Army’s Center for Military History, the Insurrection Act has therefore been used repeatedly throughout American history to help quell civil unrest—especially before the rise of well-trained (and increasingly well-equipped) modern local police forces. In virtually every case, the act was used in circumstances in which there was no serious dispute that local authorities were inadequate to the task at hand, and where domestic deployment of federal troops was seen as a means of restoring civil and civilian order, not subverting it.