Unrestricted Submarine Warfare

A merchant ship is sunk by a German U-boat during World War II. Public Domain


Unrestricted submarine warfare occurs when submarines attack merchant ships without warning rather than following prize regulations. First used during World War I, this type of warfare was highly controversial and deemed a breach of the rules of war. Resumption of unrestricted submarine warfare by Germany in early 1917 was a key reason the United States entered the conflict. Used again in World War II, it was generally accepted by all combatants though technically banned by the 1930 London Naval Treaty.


  • World War I: Germany against the Allies
  • World War II: Germany against the Allies
  • World War II: The United States against Japan
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Your Citation
Hickman, Kennedy. "Unrestricted Submarine Warfare." ThoughtCo, Jul. 31, 2021, thoughtco.com/unrestricted-submarine-warfare-p2-2361020. Hickman, Kennedy. (2021, July 31). Unrestricted Submarine Warfare. Retrieved from https://www.thoughtco.com/unrestricted-submarine-warfare-p2-2361020 Hickman, Kennedy. "Unrestricted Submarine Warfare." ThoughtCo. https://www.thoughtco.com/unrestricted-submarine-warfare-p2-2361020 (accessed June 1, 2023).