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^[1] and place crews in "a place of safety" (for which lifeboats did not qualify, except under particular circumstances)^[2] before sinking them, unless the ship showed "persistent refusal to stop ... or active resistance to visit or search".^[3] The Germans disregarded the law during the First World War, and in the most dramatic episode sank the Lusitania in 1915 in a few minutes. The U.S. demanded it stop, and Germany did so. Admiral Henning von Holtzendorff (1853-1919), chief of the 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.^[4][5]

Following Germany's resumption of unrestricted submarine warfare on February 1, 1917, 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^[6] or having them report contact with submarines (or raiders), which made them de facto naval auxiliaries and removed the protection of the prize rules.^{[7][unreliable source?][8][9][need quotation to verify][page needed]} This rendered the restrictions on submarines effectively useless.^[10] While such tactics increase the combat effectiveness of the submarine and improve its chances of survival, some^[11] 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:

1. The U-boat campaign of World War I, waged intermittently by Germany between 1915 and 1918 against Britain and her allies. One of The most brutal acts was on May 7, 1915 when U-boat U-20 deliberately torpedoed the British Cunard Luxury Liner RMS Lusitania. Germany's acts eventually brought United States into the war in 1917 on the British side, together with the Zimmermann Telegram. It was also a casus belli for the entry of Brazil into the war in 1917.
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 Baltic Sea Campaigns 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 in the Baltic Sea.
4. The Pacific War during World War II, between 1941 and 1945, waged by the United States and the British Empire 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.

• Submarine warfare
• Defensively equipped merchant ship
• Commerce raiding
• Tonnage war
• Arabic pledge
• Sussex pledge
• Tsushima Maru
• War Order No. 154
• Laconia incident
• Laconia Order
• List by death toll of ships sunk by submarines
• Baralong incidents
• Karl Dönitz

• Ronzitti, Natalino (1988). The Law of Naval Warfare: A Collection of Agreements and Documents With Commentaries. Martinus Nijhoff. ISBN 978-90-247-3652-2.
• Willmott, H. P. (2003). World War I. Dorling Kindersley. ISBN 978-0-7894-9627-0.