Post 9/11 has resulted in an increase in security and monitoring activites when it comes to internal travel within the Americas. In 2004 the Intelligence Reform and Terrorism Prevention Act created a program call the Western Hemisphere Travel Initiative which now requires US and Canadian travelers to present a passport or other legal document when they enter the US.
US citizens enter the the re-entering the US via air travel can show either a US passport or a NEXUS card. If they are entering by land you may show a US passport, US passport book, NEXUS, FAST or SENTRI card, some entry points a drivers license, and at limited points a federal issued ID with the appropriate CAC card. For sea points, you can show passport book or card, NEXUS card, state drivers license, Merchan Mariner ID, and a military ID. Please note for sea entry, if you are coming on a cruise from the Carribean a state issued ID and a birth certificate are acceptable.
The overall goal of the program is to ensure a smooth faciliation of entry points but at the same time provide accountability and enhanced security which will allow for easy identification of travellers.