As President Donald Trump pushes the bounds of military activity on domestic soil, a debate has emerged over a nearly 150-year old law that regulates when federal troops can intervene in state issues.

About 800 National Guard troops filed into Washington, D.C., on Tuesday after President Donald Trump said — without substantiation — that they were needed to reduce crime in the “lawless” national capital. Thousands of miles away, a judge in California is hearing arguments about whether the president's recent decision to federalize Guard personnel in Los Angeles during protests against immigration raids violated federal law.

Trump has also created militarized zones along the U.S.-Mexico border as part of a major shift that has thrust the army into immigration enforcement like never before.

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