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 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 rendered the restrictions on submarines effectively useless. 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.

Instances
There have been four major campaigns of unrestricted submarine warfare:
 * 1) The U-boat campaign of World War I, waged intermittently by Germany between 1915 and 1918 against Britain and her allies. This warfare was ostensibly the casus belli for the United States and Brazil's entry into the war in 1917, along with the Zimmermann Telegram for the United States.
 * 2) The Battle of the Atlantic during World War II. Between 1939 and 1945, it was waged between Germany and the Allies and from 1940 to 1943 between Italy and the Allies.
 * 3) The Naval War on the Eastern Front, during World War II between 1941 and 1945, especially from 1942. Waged by Germany and the USSR against each other, primarily on the Baltic Sea.
 * 4) The Pacific War during World War II, between 1941 and 1945, waged by the United States against Japan.

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.