CBP Publishes Informative Publication about Western Hemisphere Travel Initiative
The WHTI requirements for air travel went into place on January 2007. All U.S. citizens and nonimmigrant foreigners are required to show a valid passport or proof of citizenship when leaving or entering the U.S. from within the Western Hemisphere at an airport of entry. The program was implemented in 2009 for entry into the U.S. by land and sea.
Learn more online at https://www.cbp.gov/travel/us-citizens/western-hemisphere-travel-initiative.