What Turned the Tide in the Battle of the Atlantic?

Without success in the Battle of the Atlantic, there would have been no Second Front, no convoys to the USSR and victory against Japan would have been more difficult. The survival of Britain itself, as a protagonist in the fight against Nazi Germany, depended on winning the struggle in the Atlantic. The battle was unrelenting and the only one which lasted the entire war, with sinkings on the first and last day of WWII in Europe. 

Britain’s position as an island nation probably saved it from invasion in 1940. This strength though was also a vulnerability when it came to sustaining itself during wartime. Britain imported the greater proportion of its food, for example 87 percent of cereals and a huge percentage of raw materials used to sustain the war effort, such as iron. All of this had to be carried on merchant ships. The threat to this was the Kriegsmarine, aided by the Luftwaffe. By the end of 1941 imports, excluding oil, had fallen to half pre-war levels. Britain was in danger of being slowly defeated by the enemy severing its economic lifelines. Over 60% of the losses to the British merchant marine were caused by U-boats. Defeat of these was essential if the battle was to be won. 

How Was the Battle Fought?

The Allied convoys, especially those between North America and Britain, were the focal point of the battle. Germans tried to locate them and then use their ‘Wolf Pack’ tactics to attack. The Allies defended convoys with close escorts, separate hunt groups and air power. As well as raw courage the battle was one of leap-frogging technologies. For example, German acoustic homing torpedoes were diverted by Allied ships towing acoustic devices, named Foxer. The U-boats Schnorchel allowed them to stay submerged for long periods, evading detection by enemy aircraft. 

When Did the Scales Tip Toward the Allies?

The first part of Spring 1943 was one of increasing success for the U-boats as they began to replicate their achievements of 1942. In March they sank over half a million tons of merchant shipping, one of the highest monthly totals, but this fell by 50% in April and dropped again in May. U-boat losses were rising and peaked in the second quarter of 1943. On 24th May Germany withdrew its submarines from the North Atlantic, although they did return. 

Allied technology was temporarily in the ascendant, with high-definition radar and High Frequency Direction Finding used to locate submarines. The battle of the codebreakers was highly important and between the Allies Ultra and German B-Dienst (naval intelligence) it was about honours even during this decisive phase of the battle. However, most historians think that the breaking of the German Enigma system was of greater importance as it breached not just naval code but all German high command communication. 

The battle was essentially a tonnage war. To win, the Allies had to build more merchant ships than the Axis were sinking. The basic measure was Allied tonnage sunk versus U-boat losses and this shows that merchant ship sinkings peaked during the last quarter of 1940, achieved with an average of just 25 U-boats in the Atlantic. U-boat numbers increased steadily after this, until late Spring 1943. Merchant ship sinkings declined steadily until December 1941 when the USA entered the war and the U-boats enjoyed their second ‘happy time’. This declined in the second half of 1942 as the US introduced counter-measures. After this point sinkings continued to fall. U-boat numbers did decrease but even at the end of the European war there were still 150 operational, but their effect by then was negligible. 

The overriding reason for Allied success was the industrial capacity of the USA, building both merchantmen and warships rapidly and in huge numbers. The record for Liberty ship construction was less than 5 days and at its peak the USA was launching two a day. By the late summer of 1942 the US was building more merchantmen than the Axis were sinking and by the autumn there was a net gain in Allied merchant shipping. 

Conclusion

Germany had a chance to win this battle during the first 30 months of the war, but Britain survived, although imports declined throughout 1942. After this, the US increasingly began to impact the battle. In retrospect, so long as Britain could survive until the US entered the war and put its huge economy on a wartime footing, the Allies would eventually triumph.