 As a result of the September 11, 2001, attacks, the United States began implementing a range of measures to increase the security of its borders and its identity documents.One result of this was the Western Hemisphere Travel Initiative (WHTI), which mandates as of 2007 that a smaller, more secure number of documents verifying both identity and citizenship be used to facilitate identification and international border crossing.Previous to the WHTI, many different types of documents were acceptable to cross the border, including birth certificates issued by thousands of different authorities within the United States and Canada.