Battle of the Atlantic is the name given to the conflicts in the Atlantic Ocean throughout World War II. Britain needed to import over 1 million tons of supplies every week to feed and equip its population and war machine.
The Mining Threat
Much of the early German action involved mining convoy routes and ports. The U-boat fleet was small initially. A breakthrough came in November 1939 when a German magnetic mine was recovered intact from mud flats in the Thames estuary, allowing the British to develop countermeasures including degaussing.
The Happy Time
Admiral Doenitz advocated the wolfpack system. He calculated 300 Type VII U-boats could knock Britain out of the war. The early Type VII operations were spectacularly successful, and the U-boat crews called this the "happy time." The Royal Navy introduced convoys which concentrated warships near the U-boats.
Atlantic Operations
With the fall of France, the Kriegsmarine gained direct access to the Atlantic. The Type VIIc design arrived in large numbers in 1940 — eventually 585 were delivered. Improvements included forward-firing depth charges (Hedgehog), improved direction-finding radio equipment, and convoy escort carriers.
Operation Drumbeat
After the US entered the war, Doenitz sent U-boats to the US east coast in Operation Drumbeat (Paukenschlag). With only five Type IX boats initially, the Germans achieved spectacular results against unescorted American shipping.
The Turning Point
The RN slowly gained the upper hand through 1941–1943 with improved sonar, air cover, codebreaking, and escort tactics, eventually winning the campaign.