Whether you enter the U.S. at an airport or cross the border on foot or vehicle from Canada or Mexico, you must produce a valid passport from your country of origin. If you're a U.S. citizen, you must produce a valid U.S. passport to return to your country.
There are many ways to get around the U.S.; there are numerous airports in every state, Amtrak provides train service from coast to coast and everything in between, and car rental companies provide travelers with vehicles. To use these services, travelers must provide valid identification of some kind. If you're not a U.S. citizen, a passport will typically be your best option.
Though you may be asked for a passport as proof of identity if you're not a U.S. citizen in situation such as those described above, you don't need a passport to travel from state to state. There are no passport control stations at state lines.
