Unrestricted submarine warfare

Unrestricted submarine warfare is a type of naval warfare in which submarines sink vessels such as freighters and tankers without warning, as opposed to attacks per prize rules (also known as "cruiser rules"). Prize rules call for submarines to surface and search 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 showed "persistent refusal to stop ... or active resistance to visit or search".

Following Germany's resumption of unrestricted submarine warfare on February 1, 1917, during 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 or having them report contact with submarines (or raiders), which made them de facto naval auxiliaries and removed the protection of the prize rules. This rendered the restrictions on submarines effectively useless. While such tactics increase the combat effectiveness of the submarine and improve its chances of survival, some regard them as a breach of the rules of war, especially when employed against neutral vessels in a war zone.

There have been four major campaigns of unrestricted submarine warfare:

The four cases were attempts to navally blockade countries, especially those heavily dependent on merchant shipping to supply their war industries and feed their populations (such as Britain and Japan), even though the countries waging the unrestricted submarine warfare were unable to institute a typical naval blockade.

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