For the first time since World War II—and for the first time ever outside a formally declared war—an American president has invoked the Alien Enemies Act of 1798 to detain and deport noncitizens.  Enacted during the Quasi-War with France, the Act’s operative language remains largely unchanged today.  In March 2025, President Donald J. Trump proclaimed that members of the Venezuelan gang Tren de Aragua present in the United States are “alien enemies,” asserting that their activities, allegedly coordinated with President Nicolás Maduro’s government, constitute an “invasion” or “predatory incursion” within the meaning of the statute.  Litigation challenging the invocation of the Alien Enemies Act on the merits is pending before a number of federal courts, and may reach the U.S. Supreme Court in time.

This litigation, together with the Supreme Court’s direction that statutes be interpreted according to their original meaning, makes possible—and necessary—the first comprehensive account of the Alien Enemies Act as it was understood at enactment.  Drawing on legal, diplomatic, political, and intellectual history, this Article reconstructs the statute’s background in the law of nations, Anglo-American common law, and eighteenth-century British and American practice.  It shows that by 1798, “alien enemies” was a well established legal status, arising categorically based on nationality during a state of war.  The Act codified that settled understanding; it did not create a broader or more flexible concept tied to individual dangerousness, as the Trump administration now contends.  Through a close examination of the historical record, this Article concludes that the Trump administration’s 2025 invocation of the Act is unlawful.  There has been no “invasion” or “predatory incursion” as those terms were understood in 1798, and no one presently in the United States can properly be classified as an “alien enemy” under the statute’s original meaning.  The Alien Enemies Act is not an instrument for restraining dangerous individuals outside of wartime—a function governed instead by criminal law and immigration law.

While this Article addresses and answers questions of current importance, its scope and legal historical analysis also aim for broader significance.  Enacted amid intense partisan conflict and a grave international crisis, the Alien Enemies Act’s story illuminates enduring constitutional and legal issues, including the division of war powers between Congress and the president; the important role of Congress in interpreting the U.S. Constitution; the complex inheritance in early American law of the English common law, British governmental practices, and the law of nations; the historical scope of congressional authority over immigration and deportation; and, during the founding era and early republic, the availability of habeas corpus and judicial review in national security cases and the status and rights of noncitizens in wartime.