While the Blitz raged, German aircraft and U-boats caused heavy losses of merchant shipping in the north-western approaches to Britain. By March 1941 the situation was seen as so severe that Churchill issued his 'Battle of the Atlantic Directive' and directed all government attention towards the Atlantic.
In May 1941 in the Denmark Straits, the German battleship Bismarck and the heavy cruiser Prinz Eugen destroyed HMS Hood (the pride of the Royal Navy) with a massive magazine explosion, and escaped. Three days later the Royal Navy sunk the Bismarck while it was attempting to reach the safety of Brest. The Battle of the Atlantic continued.
The first few months of 1942 were disastrous for Britain at sea. The entry of America into the war allowed the Germans rapidly to redeploy their U-boats from the North Atlantic convoy routes to American waters. They found that the ships the British had painfully escorted across the Atlantic to Halifax (the 'Sydney' and 'Nova Scotia') were completely undefended by the Americans.
The Germans then pulled off a propaganda coup by withdrawing their capital ships from Brest through the English Channel to Germany. Uncoordinated and frantic British naval and air attacks followed but the German ships were not sunk.
From mid-1942 to May 1943 the fighting in the Atlantic was savage. The Allies made excellent use of intelligence and introduced new weapons and sensors - especially anti-submarine aircraft. The result was defeat for the U-boats. In late May 1943 after heavy losses, the Germans withdrew from the Atlantic and lost their advantage. Allied sea communications were now secure and preparations for the liberation of France went ahead.
The German invasion of Russia on 22 June 1941 was a mixed blessing for the British. They now had another ally, but an unco-operative one that demanded massive support from the stretched British economy. In August 1941 the first convoy of aid left Britain for Russia and by the end of the 1941, eight convoys had been dispatched.
It would be wrong, however, to think of Britain as standing alone between July 1940 and June 1941 without outside support. Limited support was received from America. First came the 'destroyers for bases' deal in September 1940, which was followed by the lend-lease package of March 1941 that poured equipment into Britain.
With Britain secure by land, sea and air and with men to use, ships, munitions and aircraft were produced and sent to the Middle East, the Mediterranean and the Far East. Finally Britain was the launch pad for the liberation of Europe in Operation Overlord.