Why as of June 1st 2009, the U.S. law requires all travelers including Canadian citizens to present a valid passport or other approved secure document to enter the United States?
Due to the growing threats to the U.S borders, this action is geared towards strengthening the U.S. border security that enables the Department of Homeland Security to quickly and reliably identify a traveler from these standardized documentation. Additionally, passport scanning will give the Department of Homeland Security direct access to the Canadian record database. Thus the U.S. can get access to what record an individual has in Canada just by scanning his or her passport. The Canadian Passport makes it very easy for the Department of Homeland Security officials to request a download of your criminal record from the Canadian database. Prior to June 1st 2009, the U.S. Border Protection Officers still had access but it took a longer time to check, however, they can check on someone very quickly now.
