President Trump is invoking an 18th-century wartime law that will allow the federal government to detain or deport people who are natives and citizens of countries deemed foreign adversaries — a move that's seen as part of his sweeping immigration crackdown effort.

The Alien Enemies Act — part of the Alien and Sedition Acts that Congress adopted in 1798 — gives the federal government additional authority to regulate non-citizens in times of war. It has been used just three times in the past, during the War of 1812 and World Wars I and II, as the Congressional Research Service detailed in a February report.

Largely intended to combat enemy espionage, it........

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