<p><strong>Definition: </strong></p><p>Unrestricted submarine warfare occurs when submarines attack merchant ships without warning rather than following prize regulations. First used during <a href="https://www.thoughtco.com/world-war-i-overview-2361419" data-component="link" data-source="inlineLink" data-type="internalLink" data-ordinal="1">World War I</a>, 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 <a href="https://www.thoughtco.com/world-war-ii-overview-2361501" data-component="link" data-source="inlineLink" data-type="internalLink" data-ordinal="2">World War II</a>, it was generally accepted by all combatants though technically banned by the 1930 London Naval Treaty.</p><p><strong>Examples:</strong></p><ul><li>World War I: Germany against the Allies</li><li>World War II: Germany against the Allies</li><li>World War II: The United States against Japan</li></ul>