The Western Hemisphere Travel Initiative (WHTI) is the implementation of the 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 was mandated by 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. According to the United States Department of Homeland Security, the purpose is to strengthen border security and facilitate entry into the United States for both U.S. citizens and foreign visitors.[1]