After the fall of France on 17 July 1940, only Britain and her Commonwealth allies were left to challenge the Axis (Germany and Italy). With the German occupation of Western Europe, Britain was completely dependent upon supplies and resources shipped from North America. As a result, hundreds of merchant vessels were organized into convoys that departed from the ports of Halifax and Sydney, Nova Scotia. The Royal Canadian Navy (RCN) accompanied the convoys until they reached the mid- Atlantic, whereupon the Royal Navy assumed escort duties for the remainder of the route. Eventually, the RCN provided escort duty from ports in Canada and Newfoundland to Londonderry in Northern Ireland.
Canadian tankers and RCN escorts were also deployed on the "oil run" from Venezuela and Trinidad to Halifax. After the German invasion of the Soviet Union on 22 June 1941, Canadian merchant ships and RCN escorts were used on the "Murmansk run" from Britain to the northern port of Murmansk in the Soviet Union. Once the United States had entered the war (7 December 1941), the RCN also provided escorts for convoys that travelled from New York and Boston to Halifax, which was the main assembly area for all Allied convoys bound for British ports.
All convoys were classified according to speed and destination. The prefix "H" (Homebound) designated convoys bound for Britain, while "O" (Outbound) referred to convoys returning to Halifax or Sydney. The "HX" designator indicated fast convoys that carried munitions, oil, and other explosive cargo. All slow convoys, regardless of destination, were given the prefix "SC." Convoys from British ports to Murmansk were given the prefix "JM," while those that sailed from Nova Scotia via Iceland to Murmansk were designated by the prefix "PQ." The prefix "M" designated convoys sailing to the Mediterranean. Ideally, a convoy of 60 merchant ships would have an escort of at least 2 destroyers and 4 corvettes, a ratio of 1 escort for every 10 merchant vessels. Due to the critical shortage of destroyers and corvettes early in the war, however, the first convoys had only 2 RCN escorts.
