Over twenty years passed since American and Canadian forces arrived in Afghanistan to start the War on Terrorism as a response to 9/11, and during this period, there has been an enormous controversy about the nature and the scope of the Afghanistan intervention. Amidst this controversy, this dissertation aims to unravel a strategic puzzle: why and how the United States and Canada adopted similar engagement levels, especially similar counterinsurgency (COIN) strategies between 2005/2006 and 2011. During this time, the United States and Canada fought against insurgent groups, sought to maintain stabilized areas by mentoring Afghan forces, and invested in infrastructure and governance. These goals, which corresponded to the 'clear,' 'hold,' and 'build' COIN components, entailed sending troops and civilian officials to a war zone and committing financial resources. According to structural realism as a dominant IR theory, the similarity of American and Canadian engagements constitutes a puzzle because this theory would expect that countries with different relative standings in the international system are meant to adopt different foreign and security policies. To unravel this puzzle, this dissertation uses neoclassical realism. Like a realist theory, it examines the effects of relative material capabilities on foreign policy, which is an essential factor in comparing American and Canadian foreign and security policies. Yet, unlike structural realism, it considers dimensions other than states' relative standing, such as perceptions and domestic politics, which, along with the relative standing, help to unpack the puzzle above. Based on this theory, the central argument of this dissertation is that the similarity of American and Canadian engagements resulted from similar systemic stimuli from the post-9/11 strategic environment, foreign policy executives' (FPE) similar strategic beliefs, and comparable abilities to mobilize domestic resources. By showing the occurrence of factors on both American and Canadian sides bringing about the outcome above, this dissertation seeks to undermine the ideas of the "Americanization" of Canadian foreign and security policy and the syndrome of "parochialism" of the United States regarding its Northern neighbour.