When Hitler invaded Poland on 1 September 1939, Germany was not ready for a major war at sea. The German surface fleet consisted of no more than 2 old battleships, 2 battle-cruisers, 3 pocket battleships, 8 cruisers and 22 destroyers. A few heavy ships were still building; but only two battleships and one cruiser were completed during the war. More surprising still, no preparations had been made for a prolonged U-boat campaign. The U-boats had been the most serious menace to Great Britain in the First World War; subsequent technical developments had further increased the efficiency of under-water weapons; yet only 57 German U-boats had been built by 1939, and only 26 of these were suitable for Atlantic operations.

This was not the Navy which the German Naval Staff had hoped to command in a war against Great Britain. There was neither the battle-fleet with which Admiral Raeder, the Commander-in-Chief, had thought, one day, to challenge British sea-power, nor the U-boat force which Admiral Doenitz, Flag Officer U-boats, regarded as indispensable for a German victory.

In the autumn of 1938, in preparation for a future war against Great Britain, Raeder had made plans by which Germany would have, if not a large, at least a well-balanced fleet within a reasonable period; Doenitz had taken care to see that these plans provided for an increase in the number of German U-boats. As these plans stood at the beginning of 1939, the German navy, including the fleet in being and the ships already building, was to consist of 8 battleships, the 2 battle-cruisers, the 3 pocket battleships, 16 cruisers, 2 aircraft-carriers and about 190 U-boats by the end of 1944. Further additions were to produce a total fleet of 8 battleships, 2 battle-cruisers, 3 pocket battleships, 33 cruisers, 4 aircraft-carriers and about 270 U-boats by 1948. But Raeder was forced to modify these plans in the Spring of 1939, when increasing international tension suggested that war might break out earlier than had been expected. He was forced to abandon them altogether when war did break out, in spite of his hopes, in the autumn of that year.