Western Hemisphere Travel Initiative

The Western Hemisphere Travel Initiative (WHTI) is the implementation of a requirement to show a passport or other acceptable document to enter the United States, for nationals of certain North American jurisdictions who were previously exempt from it when traveling within the Americas. The requirement comes from the Intelligence Reform and Terrorism Prevention Act, enacted in 2004 in response to the terrorist attacks of September 11, 2001, and implemented in phases between 2007 and 2009.

Source: Wikipedia — Western Hemisphere Travel Initiative (CC BY-SA 4.0)