Strategic perceptions of the Canadian North changed several times during the twentieth century, influencing the intensity and degree of military presence. Initially, the region was simply ignored. By the mid-1930s, it was perceived as a strategic barrier more formidable than either the Atlantic or Pacific Oceans. During the Second World War and the Cold War, with the views of the United States in the dominance, the area was seen as an approach—initially to Europe and Asia, and later to the heartland of North America. In contemporary Canada, the North is seen as having intrinsic value, and as such deserves to be watched over, protected and, if necessary, defended. By analyzing the interplay between defence, protection of sovereignty, and national development, this book reveals the myriad roles of the Canadian Forces who were assigned military responsibility to be Custos Borealis—Keeper of the North.