The Baltic Sea Campaigns were conducted by Axis and Allied naval forces in the Baltic Sea, its coastal regions, and the Gulf of Finland during World War II. After early fighting between Polish and German forces, the main combatants were Germany and Finland, opposed by the Soviet Union. Sweden's navy and merchant fleet played important roles, and the British Royal Navy planned Operation Catherine for the 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 one of the most densely mined waters in the world.