Western Hemisphere Travel Initiative
The Western Hemisphere Travel Initiative (WHTI) "will require all U.S. citizens, Canadians, citizens of the British Overseas Territory of Bermuda, and citizens of Mexico to have a passport or other accepted secure document to enter or re-enter United States by January 1, 2008." 
"The Intelligence Reform and Terrorism Prevention Act of 2004 (IRTPA, also known as the 9/11 Intelligence Bill), signed into law on December 17, 2004, mandated that the Secretary of Homeland Security, in consultation with the Secretary of State, develop and implement a plan to require U.S. citizens and foreign nationals to present a passport, or other secure document when entering the United States." 
Passport & Travel Information
- "Travel Spotlight," U.S. Customs and Border Protection (CBP) website.
- "Travel," U.S. Department of State website.