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In both World Wars Germany attempted to sever the supply lines across the Atlantic to the British Isles, cutting off Britain from her empire and her eventual ally in the United States. To do this they used aircraft and surface raiding ships but above all submarines (referred to as U-boats) to torpedo and mine merchantmen which were formed into defensive groups known as 'convoys' and accompanied by a variety of escort warships and aircraft. At the beginning of World War 2 German submarines had considerable success against Britain but losses were kept to an acceptable level. However the fall of France and Norway coupled with the Irish Free State's neutrality created a catastrophic situation for the Allies, allowing the Germans to base surface ships, submarines and aircraft there thus massively extending their range and endurance whilst forbidding the Allies from doing the same on Ireland's southern coast to counter them (a post war Admiralty survey estimated that Southern Ireland's neutrality cost the Allies over 300 ships and their cargoes and 5000 lives) . This was combined with the entry of the Italian fleet on the side of the Axis and the loss of the French and other occupied countries' navies and merchant fleets to the Allied cause. The war would then expand into the Mediterranean, Arctic convoys supplying the USSR and eventually into the Far East against Japan.

Allied ships would group together in convoys making them much harder to find in the vastness of the oceans. If they had been spread out and sailing individually it allowed a U-boat to torpedo one ship and then wait for the next to come along. If a U-boat sighted a convoy it might attack and sink one or two ships but the rest of the convoy would then outrun it, limiting losses to a large degree. To counter this the Germans used long range reconnaissance aircraft ('Focke-Wolf Condors') to locate and sometimes attack convoys. They also spread lines of U-boats across the Atlantic and when one sighted a convoy it would tail it and report its' position back to German high command (as we witness when Compass Rose sinks the U-boat it catches on the surface). Other U-boats would then be directed to intercept the convoy en masse and overwhelm its' defences in what became known as a Wolf Pack. Escorts would ring the merchantmen and use sonar ('Asdic') to locate U-boats and attack them with underwater bombs known as depth charges and occasionally by ramming. Whilst the priority was not to destroy U-boats but protect the merchant ships eventually the Allies had enough escorts to form hunting groups, leaving one group of escorts to protect the convoy whilst another pursued the enemy. German U-boats and aircraft would also sow mines around Allied ports whilst the Allies air forces would relentlessly bomb U-boat bases but to little effect and often huge losses.

The decryption of German naval codes by Bletchley Park ('Enigma') allowed many convoys to be routed safely around the U-boats but this was offset by the Germans also breaking Allied naval codes. The introduction of long range land based aircraft and escort carriers (merchant ships converted into mini aircraft carriers carrying a handful of planes) allowed convoys to have air cover across the entire Atlantic, giving them protection from German aircraft and U-boats (it was not necessary for planes to actually sink enemy submarines, once they were driven underwater by air patrols the convoy could simply outrun them). Advances in radar and radio direction finding ('Huff-Duff') allowed escorts to pinpoint U-boats whilst Italy was knocked out of the war in 1943 and the liberation of France in 1944 denied the German navy their ports there. Ultimately however ship production by Allied shipyards out produced the Germans (one pre-fabricated 'Liberty ship was constructed in only 5 days) meaning that the Axis were never able to sink enough to make a difference.

The Allies lost over three and a half thousand ships and over 70,000 personnel in nearly six years. The Axis lost over 30,000 killed and 800 submarines sunk, the German U-boat arm having the highest casualty rates of all the German armed forces. Although a tactical victory for the Axis it was a strategic victory for the Allies.