With the outbreak of hostilities in the Russo-Ukrainian War, the arrival and participation of foreign volunteer fighters has been one of the defining characteristics of the conflict. According to a March 2022 statement by the Ukrainian Foreign Minister, Dymetro Kuleba, over 20,000 volunteers have joined the Ukrainian armed forces, of which an estimated several thousand are American citizens. [1] Despite the uncertainty about precise numbers, it is important to consider the legality under U.S. law of U.S. citizens volunteering in foreign nations. Throughout the two and a half centuries during which American volunteers have participated in foreign wars, there have been a series of acts by Congress and rulings by the Courts on the legality of such participation.

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