In Ludecke v. Watkins, the U.S Supreme Court ruled in favor of the government's power to deport enemy aliens during peacetime under the Alien Enemy Act of 1798. The case involved Karl Ludecke, a German national who was ordered for deportation by Francis Biddle, Attorney General at that time due to his affiliation with Nazi organizations before World War II. Despite Germany’s surrender in 1945 and President Truman’s official proclamation ending hostilities later that year, Ludecke remained detained for deportation proceedings which he challenged as unlawful since war had ended. However, the court held that an 'alien enemy' status continues until a formal peace treaty is signed or Congress otherwise directs - neither of which had occurred when Ludecke was ordered deported. Therefore it upheld his continued detention and subsequent deportation order.

In the dissenting opinion for Ludecke v. Watkins, Justice Frank Murphy argued that the majority's interpretation of the Alien Enemy Act was overly broad and potentially dangerous. He contended that this interpretation could allow for indefinite detention of alien enemies even after hostilities had ended, which he believed to be a violation of basic principles of justice and liberty. Furthermore, he pointed out that there were other legal mechanisms available to deal with individuals who posed a threat to national security without resorting to such extreme measures. In his view, it was not enough for Congress simply to declare war; they also needed explicit authorization in order for detentions like Ludecke's to continue beyond the cessation of active hostilities.