When World War II started in September 1939, the German Kriegsmarine were not prepared to challenge the allied naval forces. Unlike the other arms of the German military, the naval construction plan (Z-Plan) had only started a few months before. On September 1, 1939, the navy still had a total personnel strength of only 78,000.
In the early years, the Kriegsmarine achieved some remarkable results, and participated in the Battle of Westerplatte, Battle of the Danzig Bay during the Invasion of Poland. In 1939, major events for the Kriegsmarine were the Battle of the River Plate, the sinking of the Battleship HMS Royal Oak, and the sinking of the Aircraft Carrier HMS Courageous. The Battle of the Atlantic started in 1939, although the German submarine fleet was hampered by the lack of good ports from which to attack Allied shipping.
The German U-boats would continue their successful operations until May 1943. But due to Allied anti-submarine tactics and better equipment the U-boat fleet started to suffer heavy losses. Radar, longer range air cover, improved tactics and new weapons all contributed.
From late 1944 until the end of the war, the surface fleet of Kriegsmarine was heavily engaged in providing artillery support to the retreating German land forces along the Baltic coast and in ferrying civilian refugees to the western parts of Germany. Large parts of the population of eastern Germany fled the approaching Red Army out of fear for Soviet retaliation. The Kriegsmarine evacuated large numbers of civilians in the evacuation of East Prussia and Danzig in January 1945.
At the end of the war, only two of the major Kriegsmarine ships were still operational, all other were sunk, either during their operations or destroyed during the last months and weeks of the war in their bases.