"Canada and the United States closely cooperated in most security issues during the 20th century. In recent years, however, security relations between Canada and the United States have become strained, mainly due to disagreements on the methods used by the United States in prosecuting the Global War on Terror. The first policy issue was the Canadian government's decision to decrease security resources significantly in the wake of the Cold War. The second issue centers on Canada's disagreement concerning Operation IRAQI FREEDOM, as well as other U.S. policy decisions such as the use of Guantanamo Bay."
Strategic Studies Institute of the U.S. Army War College: http://www.strategicstudiesinstitute.army.mil/