In Klapprott v. United States, the Supreme Court ruled that a naturalized citizen could not be stripped of their citizenship without due process of law. The case involved Carl Wilhelm Klaprott, who was accused by the U.S government of participating in Nazi activities and failing to disclose this information during his naturalization proceedings. The government sought to revoke his citizenship through denaturalization proceedings but did not provide him with an opportunity for a hearing or trial as required under the Fifth Amendment's guarantee of due process rights. In its decision, the court held that such actions violated constitutional protections and emphasized that once granted, American citizenship is a right rather than a privilege subject to revocation at will by governmental authorities.

In the dissenting opinion for KLAPPROTT v. UNITED STATES, 1948, Justice Black argued that Klaprott's right to due process was violated because he wasn't given an adequate opportunity to defend himself against the denaturalization charges. He contended that a person should not lose their citizenship without a full and fair hearing where they can present evidence and confront witnesses against them. The justice also expressed concern about using denaturalization as punishment for political beliefs or affiliations, arguing it could lead to abuses of power by the government. Furthermore, he disagreed with treating denaturalization proceedings like civil cases instead of criminal ones since losing one’s citizenship is such a severe penalty; thus requiring higher standards of proof than in ordinary civil actions.