Fact File : Battle of the Atlantic
1939 to 1943
Location: Atlantic Ocean
The US, British and Canadian navies versus the Axis navies, particularly German U-boats.
Outcome: The worst of the battle was over when the Germans called a halt to major U-boat operations in the Atlantic on 23 May 1943, but the threat to Allied shipping was never entirely eliminated.
The term 'Battle of the Atlantic' was coined by Winston Churchill to describe the protracted struggle by the Allies to secure shipping routes across the Atlantic. The Allies' main objectives were to blockade the Axis powers (limiting productivity and diminishing morale), to secure their own shipping routes, and to wage war overseas without any impediment.
As a struggle, the Battle of the Atlantic is symbolic of the scale of the global war in which nations had to fight against the enemy on land or sea thousands of miles from home. The successful transportation of troops and materials was as crucial as battle itself.
Only after the war did Churchill confess that it was the Atlantic that caused him most concern:
'The only thing that really frightened me during the war was the U-boat peril.'
(He was referring to German submarines, which remained the Allies' principal threat at sea). The greatest challenge, Churchill felt, was to manage strategy around the Atlantic shipping routes which required 'statistics, diagrams and curves unknown to the nation, incomprehensible to the public'.
Britain had the largest fleet in the world - 3,000 ocean-going vessels and 1,000 large coastal ships. They required 160,000 men to man them. The German navy, in contrast to the submarine fleet, was in poor condition following World War One and initially the Germans underestimated the role the U-boat might play - only 46 vessels were in operation, intended for surface attacks.
The British navy was successful in sinking the pocket battleship Graf Spee in December 1939 and the battleship Bismarck in 1941, but from the summer of 1940 the U-boat menace began to grow. Britain faced problems - the air gap in the Western Atlantic meant that the RAF could not fully patrol U-boats. The Allied occupation of Iceland (belonging to German-occupied Denmark) was an advantage, but long-range aircraft had to be developed before the air gap could be conquered. The Canadian navy eventually assisted Britain in covering this gap.
The Battle of the Atlantic really gained pace after 1941 when the U-boat captains began to expand operations further. Admiral Dönitz, the German commander of the U-boats, developed a strategy known as the 'wolfpack', in which U-boats would close in on the enemy at night.
The British Navy had previously placed much faith in Asdic (an early form of sonar) to detect submerged U-boats, and this way they were able to counter the surface threat they posed, but Asdic was not effective against the wolfpack manoeuvre.
The conquest by Germany of Norway and France gave the Germans forward bases, increasing the U-boats' range and enabling long-range aircraft to patrol over the Atlantic, carrying out reconnaissance for the U-boats. As the U-boats became more successful they were put into wider use.
The British were consequently forced to divert their own shipping away from vulnerable UK ports, and needed to provide naval escorts for convoys for greater stretches of the journey to North America. Churchill sought help and negotiated the destroyers-for-bases agreement with the US administration, adding a further 50 naval destroyers in exchange for access to British bases, predominantly in the West Indies. America then entered the battle in May 1941 and took over escort duties in the western Atlantic, beginning a shooting war with Germany that resulted in their first loss - the US destroyer Reuben James was torpedoed and sunk by the submarine U-562.
The Atlantic battle changed again with the German invasion of Russia and following Pearl Harbor and the entry of Japan into the war. This increased the scale of the war and Japan was America's primary threat now. But America's response was surprising; they failed to set up coastal convoys and blackout towns, and the German U-boats enjoyed what is darkly known as 'happy time', destroying vast amounts of coastal shipping.
The real crisis came in early 1943. Britain was running out of fuel and the number of operational U-boats had increased from 47 to 200. The Allies' greatest weapon became radio intelligence and the ability to intercept the German Enigma code so that U-boat manoeuvres could be anticipated. This intelligence (Ultra) saved the situation, along with aggressive anti-submarine tactics, better weapons and the development of long-range aircraft (Liberator) equipped with radar.
By April 1943 the U-boats were clearly struggling to make an impact and Allied destruction of German submarines began to escalate: 45 were destroyed in April and May. Dönitz decided to put a halt to U-boat operations on 23 May 1943. Had the Germans succeeded in producing their new types of super-submarines, the Types XXI and XXIII (which were being tested in the Baltic even as German defeat looked inevitable), they would have proved an even greater threat, possibly reversing the outcome of World War Two.
The fact files in this timeline were commissioned by the BBC in June 2003 and September 2005. Find out more about the authors who wrote them.