This thesis asks why was only a maritime warning mission added to the binational North American Aerospace Defense Command’s (NORAD) existing aerospace warning and control mandate and not other missions? There were other viable options that could have come from the fallout of 9/11 to improve security and defence of North America such as maritime surveillance, maritime control, not to mention land and the cyberspace domains. In the end, NORAD was assigned only maritime warning in 2006. Using the international relation theory of functionalism as a framework to understand Canada-US defence policy decisions and joint defence initiatives, the maritime warning mission addition can be partially explained but it is not sufficient. By investigating from the Cold War era when NORAD was established until the post 9/11 era when the command was eventually signed in perpetuity, this thesis argues that functionalism alone is insufficient. It is not just a functional logic and the need for specific functional-technical solutions that underscores the origins and continued existence of NORAD as a binational organization but also the unique nature of the Canadian brand of functionalism where the Canadian military, in reality, takes the lead in shaping security decisions that favour and prioritize US decisions. This investigation concludes maritime warning was adopted by NORAD because it was the least controversial and was acceptable to Canadian and US military “professionals” in NORAD.