All U.S. citizens flying into the country from an international location must show a passport book. U.S. citizens flying home after visiting Canada, Mexico, Bermuda or the Caribbean can show a passport card, which resembles a driver's license but comes from the U.S. Department of State. Members of the military flying on official business need only present an official military identification card.
U.S. citizens flying domestically need to show any valid photo identification including a driver's license, passport book or card. Four states, as of 2010, have also begun offering enhanced driver's licenses, which emits a special radio frequency for extra security. The enhanced driver's licenses don't work for international travel.
Congress approved the passport requirements as part of the Western Hemisphere Travel Initiative (WHTI) in the wake of the 9/11 attacks. The WHTI was included in the Intelligence Reform and Terrorism Prevention Act of 2004. The government wanted to ramp up border security while allowing airport officials to easily identify travelers. The new requirements for air travel started in 2007.
