Section 53.1 - Passport requirement; definitions(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.(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) ).