Although widely ignored by political scientists and scholars of international relations alike, historically unprecedented demographic change is arguably the most powerful force to affect national security and international stability over the coming decades. Fertility, mortality and migration are the one set of variables in the social sciences that can be projected into the more distant future with a reasonable degree of accuracy. As a result, the structural constraints and possibilities of demographic trends provide traction in anticipating future developments. This article examines the evolution of Canada - United States defence and security relations. It turns out that demographic trends among allied countries are bolstering Canada's value as a US ally in both a bilateral and a multilateral context. In fact, demographic trends imply that Ottawa will be able to expand ability to punch above its weight in Washington, thus making Canada and the United States even more co-dependent in asserting their defence and security interests.