Dönitz at Sengwarden — U-boats Held in Port

In the autumn of 1939, the German U-boats had won resounding tactical successes: the Royal Oak, the Courageous, and the Athenia, sunk on 3 September 1939 with 117 dead including 28 Americans — a major diplomatic incident. But by early 1940 the submarine fleet was facing an operational crisis.

On 1 January 1940, Dönitz officially had 57 U-boats, of which 38 were operational. The bad winter weather, with its North Atlantic storms and seas often running at force 8 to 9, and maintenance constraints brought the number actually on patrol down to 15. More serious still, the G7e electric torpedoes suffered from massive defects: nearly 30% missed their target because of faulty magnetic detonators.

Dönitz, 49, had commanded the submarine fleet since 1935 as head of BdU, the Befehlshaber der Unterseeboote, and was regarded as a master of tactics. It was he who had developed the Rudeltaktik doctrine, the "wolfpack tactic", concentrating several U-boats against a single convoy. But in this winter of 1939-1940, for lack of numbers, he could not put it into practice.

Hitler demanded spectacular results for propaganda purposes, and Raeder, head of the Kriegsmarine, was pressing Dönitz to step up sorties.

Dönitz had to choose his strategic response to the winter crisis.