Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchant ships such as freighters and tankers without warning, as opposed to attacks per prize rules (also known as "cruiser rules").
Prize rules call for warships to search merchantmen and place crews in "a place of safety" (for which lifeboats do not qualify, except under particular circumstances) before sinking them, unless the ship shows "persistent refusal to stop ... or active resistance to visit or search". To follow the rules a submarine must surface, defeating the purpose of submarines and putting itself in danger of attack.
During the First World War, the United Kingdom introduced Q-ships with concealed deck guns and many armed merchantmen, leading Germany to ignore the prize rules. In the most dramatic episode they sank Lusitania in 1915 in a few minutes because she was carrying war munitions. The U.S. demanded it stop, and Germany did so. Admiral Henning von Holtzendorff, chief of the Imperial Admiralty staff, argued successfully in early 1917 to resume the attacks and thus starve the British. The German high command realized the resumption of unrestricted submarine warfare meant war with the United States but calculated that American mobilization would be too slow to stop a German victory on the Western Front.
Following Germany's resumption of unrestricted submarine warfare on February 1, 1917, countries tried to limit or even abolish submarines. The 1909 Declaration of London required submarines to abide by prize rules. These regulations did not prohibit arming merchantmen, but having them report contact with submarines (or raiders) 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.
After World War I, there was a strong push to construct international rules prohibiting submarine attacks on merchant ships. In 1922 the United States, the United Kingdom, Japan, France and Italy signed the Washington Treaty on Poison Gas and Submarines, to so restrict the use of submarines as to make them useless as commerce raiders. France did not ratify, so the treaty did not go into effect.
Interwar prohibitions on unrestricted submarine warfare have been described as being too unspecified, thus leading to disagreements over how to interpret the rules and agreements. For example, it was unclear what differentiated merchant ships from military ships, in particular given that Britain wanted to retain the rights to arm its merchants. Furthermore, it was considered impractical for small submarines to take on the crews of noncombatant ships due to a lack of space. Crews could be placed in emergency boats, but there was disagreement as to how safe that was.
Prior to World War II, 48 states had accepted the prohibitions on unrestricted submarine warfare, including the great power combatants during World War II.
There have been four major campaigns of unrestricted submarine warfare, one in World War I and three in World War II:
The four cases were attempts to impose a naval blockade on countries, especially those heavily dependent on merchant shipping to supply their war industries and feed their populations (such as Britain and Japan), when their enemies were unable to institute a conventional naval blockade.