The use of submarines during World War I made difficult for merchants to bring supplies to the Allies.
Submarine warfare during World War I was in part a struggle between the German U-Boats and the convoys of supplies across the Atlantic bound for Great Britain. The British and Allied submarines carried out a wide spectrum of operations in the Baltic Sea, the North Sea, the Atlantic Ocean, the Mediterranean Sea and the Black Sea. Only a few actions took place outside the North Atlantic. Attacks by German submarines on Allied merchants gave the Americans a direct reason to enter the war in April 1917.
It was assumed that all participants would respect the Hague Conventions of 1899 and 1907 but that was not the case with submarines. Initially the German submarines tried to comply with the Capture Rules but ended up doing underwater warfare without restrictions. American diplomatic pressure forced the Germans to contain the attacks for some time, but in January 1917 Germany established a War Zone that surrounded the British Isles and sank 25% of the ships that penetrated it, until the convoys began to count on military escort. The sinking of the Pathfinder was the first victory in combat of a modern submarine, along with the feat of the U-9, that sank three British cruisers in less than one hour, they granted to the submarine the status of a new and important component of the naval war.
German submarines were used to lay mines and to attack iron ore transports in the Baltic. The British submarine fleet in the Baltic operated protecting the Russians until the Russian-German pact was signed.