Baltic Sea campaigns (1939–1945)

The Baltic Sea campaigns were conducted by Axis and Allied naval forces in the Baltic Sea, the Gulf of Bothnia, the Gulf of Finland and the connected lakes Ladoga and Onega on the Eastern Front of World War II. After early fighting between Polish and German forces, the main combatants were the Kriegsmarine and the Soviet Navy, with Finland supporting the Germans until 1944 and the Soviets thereafter. The Swedish Navy and merchant fleet played important roles, and the British Royal Navy planned Operation Catherine for control of the Baltic Sea and its exit choke point into the North Sea.

While operations included surface and sub-surface combat, aerial combat, amphibious landings, and support of large-scale ground fighting, the most significant feature of Baltic Sea operations was the scale and size of mine warfare, particularly in the Gulf of Finland. The warring parties laid over 60,000 naval mines and anti-sweep obstacles, making the shallow Gulf of Finland some of the most densely-mined waters in the world.