In the 1943 case Mortensen et ux. v. United States, the Supreme Court ruled on a matter related to immigration law and naturalization procedures. The appellants were two Danish citizens who had entered the U.S., married American women, and fathered children with them but had not completed their naturalization process before being drafted into World War II military service abroad. They argued that they should be considered as having been "lawfully admitted for permanent residence" at the time of their departure due to their marriage status and intent to become citizens, which would exempt them from certain provisions of immigration law upon return. However, the court disagreed with this interpretation. It held that under existing laws at that time, an alien could only be regarded as "lawfully admitted for permanent residence" if he has obtained an immigration visa or equivalent document allowing him entry into America for permanent residency purposes - something neither appellant had done prior to leaving America during wartime. Therefore, despite their marriages and intentions towards citizenship before departing overseas in military service capacity; without completing proper legal processes first – they were still subject to standard rules governing re-entry after foreign travel by non-citizens.

In the dissenting opinion for Mortensen et ux. v. United States, Justice Frank Murphy argued that the majority's decision was a misinterpretation of the Nationality Act of 1940 and an infringement on individual rights. He contended that Congress did not intend to strip natural-born citizens of their citizenship without explicit renunciation or treasonous acts against the U.S., as it would be unconstitutional to do so otherwise. According to him, mere residence in a foreign country should not constitute expatriation unless there is clear evidence proving disloyalty towards America or allegiance towards another nation. Furthermore, he criticized how this ruling could potentially lead to statelessness for many individuals who were unaware they had lost their American nationality due to residency abroad - something which international law seeks to prevent.