Humanities › History & Culture Unrestricted Submarine Warfare Share Flipboard Email Print A merchant ship is sunk by a German U-boat during World War II. Public Domain History & Culture Military History Battles & Wars Key Figures Arms & Weapons Naval Battles & Warships Aerial Battles & Aircraft Civil War French Revolution Vietnam War World War I World War II American History African American History African History Ancient History and Culture Asian History European History Genealogy Inventions Latin American History Medieval & Renaissance History The 20th Century Women's History View More By Kennedy Hickman Military and Naval History Expert M.A., History, University of Delaware M.S., Information and Library Science, Drexel University B.A., History and Political Science, Pennsylvania State University Kennedy Hickman is a historian, museum director, and curator who specializes in military and naval history. He has appeared on The History Channel as a featured expert. our editorial process Facebook Facebook Kennedy Hickman Updated March 06, 2017 Definition: 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. Examples: World War I: Germany against the AlliesWorld War II: Germany against the AlliesWorld War II: The United States against Japan