PASSPORT REQUIREMENT FOR AIR TRAVEL IN WESTERN HEMISPHERE
From January 23, 2007 citizens of the U.S., Canada, Mexico and Bermuda are required to present a passport upon entering the U.S. when arrival is by air from any part of the Western Hemisphere. The only acceptable alternative documents to a passport for air travel is the Merchant Mariner Document (MMD) that is issued by the U.S. Coast Guard to U.S. Merchant Mariners and the NEXUS Air card that is issued to American and Canadian citizens, lawful permanent residents of the U.S. and Canada meeting certain eligibility requirements. The MMD card is only accepted when used on official business and the NEXUS Air card is only accepted at certain airports.The new air requirement is the result of recommendations made by the 9/11 Commission and was passed into law in the Intelligence Reform and Terrorism Prevention Act of 2004.
As of January 31, 2008, U.S. citizens and citizens of Canada, and Bermuda traveling by air between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda are required to present a valid passport or other WHTI-compliant documentation to enter (or depart) the U.S.
See also: http://www.dhs.gov/western-hemisphere-travel-initiative-basicsDorothy S.Chair
Citizenship Committee
February 2007 (Updated, January 2014, Judy Furukawa)