At no point during World War II did continental North America become an active theatre of war. Canada could not, however, let itself go undefended against potential attack from Axis powers. The threat of a sustained attack beyond the odd raid was never high, although anxiety over such an event spiked in 1939-40, when Britain herself was in danger of falling to Germany, and in 1941 after the Japanese attack on Pearl Harbour. The largest threat to Canadian interests that the Axis realistically posed was to its shipping. German U-boats terrorized North Atlantic waters, a significant problem that the Eastern Command had to deal with.
Home defence was the primary objective of the Royal Canadian Air Force (RCAF) from 1935 until the beginning of World War II. This was reflected in the original, August 1939 plan which, of 23 squadrons to be organized, 17 were to stay in Canada and only six sent abroad. While events in Europe quickly convinced Canada that more forces were needed abroad by 1941, there were 17 RCAF squadrons overseas home defence continued to be a high priority for the Canadian government, if for no one else. Canada’s perceived requirements for home defence were perpetually at odds with what their larger allies, Britain and the United States, saw as being necessary. Canadian plans for expansion of the Home War Establishment were undermined by the country's dependence on these allies for scarce aircraft that were more often dedicated to other efforts.
The Royal Canadian Air Force (RCAF) operated in three broad theatres of war overseas: Northwest Europe, the Mediterannean and Southeast Asia. By far the bulk of its activity took place in Northwest Europe, where RCAF aircrews took on a variety of roles: day fighters, night fighters, bombers, intruders, transport, pathfinding, laying mines, escorting ships and taking on U-boats in the Atlantic, among others. The RCAF’s greatest contribution to action in the Mediterannean was with the dispatch of 331 Wing, made up of three bomber squadrons, which played a part in reclaiming the island of Sicily and gaining a toehold in Italy. In Southeast Asia, Canadian transport squadrons helped support the critical supply lifeline for troops moving through the mountainous jungles of Burma.