The Battle of the Atlantic was the longest battle of the war. Its start and finish matching the start and finish of the war in Europe.In terms of its importance, the Battle of the Atlantic ranked alongside and possibly surpassed other battles whose outcomes were essential to Allied victory: the Battle of Britain, the Battle of Stalingrad, the Battle of El Alamein, the Battle of Normandy,… (One should not forget the battles that did not take place in a theatre of war, for example: the fight to convince the American public of the wisdom of Lend-Lease and the effort required to drive that legislation through the American Senate and the House of Representatives). Germany’s main weapon was the U-boat, although surface raiders should not be discounted. The Allies’ weapons were their warships and their aircraft, and the depth charges that could be dropped from both. Detection technology, such as asdic/sonar, and the efforts of British code-breakers were also of paramount importance.
Following the fall of France, Germany had the added advantage of naval bases (such as Lorient) that were on the French Atlantic coast.The Battle of the Atlantic had four phases.
Phase 1: July 1940 to December 1941. Britain was on her own. The U-boat wolf-packs enjoyed considerable success.
Phase 2: December 1940 to March 1943. The USA had entered the war and the U-boats continued to gain in strength. (There was a vast increase in targets and in coastline for the Allies to defend).
Phase 3: April to May 1943. Following the ‘climax’ of the battle in March (when Allied losses were heavy), an improvement in defences—detection, air cover, tactics—served to tip the balance in favour of the Allies. The surviving U-boats pulled back to lick their wounds.
Phase 4: June 1943 to May 1945. With improved sonar, radar and codebreaking, and many more ships and aircraft, the Allies went on the offensive and defeated the U-boats once and for all.
During the Battle of the Atlantic, more than 6,000 Allied ships and over 780 U-boats were sunk.