Unrestricted Submarine Warfare

Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchantmen without warning, as opposed to attacks per prize rules (also known as "cruiser rules"). Prize rules call for submarines to surface and search for merchantmen and place crews in "a place of safety" (for which lifeboats did not qualify, except under particular circumstances) before sinking them, unless the ship has shown "persistent refusal to stop ... or active resistance to visit or search".

Following the use of unrestricted submarine warfare by Germany in the First World War, countries tried to limit or even abolish submarines. Instead, the London Naval Treaty required submarines to abide by prize rules. These regulations did not prohibit arming merchantmen, but arming them or having them report contact with submarines (or raiders) made them de facto naval auxiliaries and removed the protection of the prize rules. This made restrictions on submarines moot. While such tactics increase the combat effectiveness of the submarine and improve its chances of survival, they are considered by some to be a breach of the rules of war, especially when employed against neutral country vessels in a war zone.