As of Jan. 31, 2008, all persons crossing land borders or sea borders into or out of the United States must show appropriate documents proving citizenship and identity.
Acceptable documents include a passport, a Trusted Traveler Card (NEXUS, SENTRI or FAST) or a military ID with travel orders. If a passport is not available, then U.S. citizens can show a government-issued photo ID (such as a driver’s license) and a birth certificate or naturalization certificate.
Border crossing personnel are allowed some latitude in enforcement (if the travelers can establish his identity clearly enough) at first.