22 CFR §53.1

1. (a)It is unlawful for a citizen of the United States, unless excepted under 22 CFR 53.2, to enter or depart, or attempt to enter or depart, the United States, without a valid U.S. passport.
2. (b)For purposes of this part “United States” means “United States” as defined in section 215(c) of the Immigration and Nationality Act of 1952, as amended (8 U.S.C. 1185(c)).