When America joined the war a few weeks later its naval command under Admiral King refused to accept the tactics the Royal Navy had spent the previous two years developing. New England coastal towns were not blacked out and King rejected the convoy system claiming a shortage of ships and insisting that poorly defended convoys just made bigger targets.
At the same time German U-Boats launched a new offensive directed at ships off the American coast: Operation Drumroll. On January 12th 1942, the unescorted British merchant ship Cyclops was the first to be sunk. By the end of the month 45 more allied merchant vessels had been destroyed in the same area. In February the number was 65.
By the start of 1943, the situation for allied shipping had reached crisis point. Admiral Donitz, promoted to Commander in Chief of the German Navy now had over 200 U-Boats at his disposal, over five times the force that had been available in the first months of 1940. British supplies were now at critical levels, particularly oil stocks, and there was doubt that British ship production could continue to keep pace with the huge losses being suffered.
But the situation was changing. Increased US naval production began to lead to better escorted convoys. Likewise, the Canadian Navy, tiny in 1939 was now one of the world's largest.
The arrival of US B-24 Liberator aircraft finally closed the mid-atlantic gap. Convoys now had air support across the entire Atlantic. More and more reliable British understanding of German codes enabled submarine packs to be avoided or targeted. Admiral Donitz personally contacted his boats regularly, giving Allied codebreakers plenty of material to work with.
In May 1943 41 U-Boats were destroyed by Allied attacks. In July and August a further 62 were destroyed.
On the 24th May 1943, Donitz ordered U-Boats to be withdrawn from the North Atlantic convoy routes.
This move allowed U-Boat numbers time to recover but it also gave the Allies more and more access to supplies to prepare for the D-Day landings in occupied Europe.
On Boxing Day 1943 the Scharnhorst, the Reich's last capital ship was sunk off the coast of Norway by British and Norwegian vessels.
The Germans desperately attempted to fight these improvements in Allied resources and intelligence with technology. U-Boats were fitted with schnorkel apparatus that allowed their batteries to be recharged without surfacing. Acoustic torpedoes targeted ships by listening for the sounds of their engines. 432 Allied Ships were sunk in 1944. But in earlier years German submarines had managed to sink such numbers in a matter of weeks. Now allied production soared as commanders prepared for D-Day on June 6th and the final assault on northern Europe.
U-Boats no longer ruled the waves and the Battle of the Atlantic was being overtaken by events elsewhere. On the 30th April 1945 the new Type XXIII submarine undertook its first mission. Tiny and highly advanced these new vessels were destined to play no part in the war. On the same day, Hitler committed suicide in his Berlin bunker.
Over the course of the six year Battle of the Atlantic some 3,500 allied merchant vessels had been destroyed with the loss of over 30,000 lives. Germany had lost 783 U-Boats and more than 28,000 sailors. Britons had had to undergo stringent rationing in order to make supplies stretch as far as possible but German attempts to starve Britain into submission had failed and her grip on the waters of the Atlantic was broken.