The Nationality Act of 1940 revised numerous provisions of law relating to American citizenship and naturalization. It was enacted by the 76th Congress of the United States and signed into law on October 14, 1940, a year after World War II had begun in Europe, but before the U.S. entered the war.

Afroyim v. Rusk, 387 U.S. 253 (1967), was a landmark decision of the Supreme Court of the United States, which ruled that citizens of the United States may not be deprived of their citizenship involuntarily. The U.S. government had attempted to revoke the citizenship of Beys Afroyim, a man born in Poland, because he had cast a vote in an Israeli election after becoming a naturalized U.S. citizen. The Supreme Court decided that Afroyim's right to retain his citizenship was guaranteed by the Citizenship Clause of the Fourteenth Amendment to the Constitution. In so doing, the Court struck down a federal law mandating loss of U.S. citizenship for voting in a foreign election—thereby overruling one of its own precedents, Perez v. Brownell (1958), in which it had upheld loss of citizenship under similar circumstances less than a decade earlier.