In the case of Knauer v. United States (1945), the Supreme Court was asked to determine whether a naturalized citizen could be denaturalized for affiliations that occurred after their naturalization process had been completed. The petitioner, William Knauer, became a U.S. citizen in 1921 but later joined organizations affiliated with Nazi Germany during World War II. As such, he was accused by the government of having obtained his citizenship illegally and fraudulently due to these actions which demonstrated lack of loyalty towards America. The court ruled unanimously against Knauer stating that if it is proven in court that an individual showed disloyalty or affiliation with enemy forces at any point after becoming a citizen - even many years later - this can serve as evidence that they were not attached to principles of American constitution when they took their oath; thus making them liable for denaturalization proceedings under Section 338(a) of Nationality Act. This ruling set precedent allowing post-naturalization conduct as admissible evidence in denaturalization cases and emphasized on importance of continued allegiance from citizens towards United States Constitution beyond just time period leading up to naturalization ceremony itself.

In the dissenting opinion for Knauer v. United States, Justice Frank Murphy argued that a naturalized citizen should not be denaturalized based on false statements made during the naturalization process unless those falsehoods were material to gaining citizenship. He believed that this case did not meet such criteria and thus disagreed with the majority's decision to strip Mr. Knauer of his American citizenship due to misrepresentations he made about his affiliation with Nazi organizations prior to becoming a U.S. citizen in 1929, long before World War II or America’s involvement in it. Justice Murphy emphasized that there was no evidence indicating these affiliations would have prevented him from being granted citizenship at the time of his application, hence they were immaterial lies and should not result in denaturalization years later.