The Western Hemisphere Travel Initiative (WHTI) is a U.S. initiative started in 2004 as a way to streamline entry to the United States from various other countries in the Western Hemisphere. As part of the WHTI, U.S. citizens can present other documents besides a passport when entering the United States. These may include a passport card, Trusted Traveler card (such as NEXUS or Global Entry). Make sure you understand the requirements published by the U.S. government to avoid a sticky situation at the border.