The Battle of the Atlantic was one of the bloodiest campaigns of World War II, fought when the Nazis had almost total domination of Europe, and Britain was the remaining surviving allied nation in Europe.
The Nazis wanted to starve the British into surrendering by cutting off their lifeline, which are made of convoys of ships sailing from the US to the British. Immediately, the Nazis dispatched U-boats, which prowls around the Atlantic to sink entire convoys of merchant ships, accompanied by just a few frigates. Soon, U-boats begin to hunt in wolf-packs, made from 10-30 U-boats, and began to realise that the convoys' aircraft escorts are deadly. Fortunately, there is an "Achilles Heel", when radar range drops out, and aircraft need to turn back. While the merchant fleets were crossing the "Achilles Heel", the U-boats made surprise attacks. Soon, U-boats found the Lawrence River the perfect place to attack ships, as the freshwater and saltwater boundary made sonar detection useless. The US then closed down the Lawrence River. However, when Britain was just about to starve, the campaign began to turn towards the Allies. New kinds of Depth charges were invented that allowed them to sink deeper before detonating, just as the U-boats were upgraded into a level where it could dive deeper. The allied continued to build countermeasures faster than the axis could create their own, and soon the Achilles Heel gap was closed, and aircraft lead to the demise of U-boats. Britain was saved, and the Allies won the Battle of the Atlantic.