The risk associated with this plan however, was that of bringing the United States into the war. The German navy used the Unterseeboot, or U-boat, to sink 5,000 ships measuring more … U.S. Germany's resumption of unrestricted submarine warfare in February 1917, together with the Zimmermann Telegram, brought the United States into the war on the British side. Definition: Unrestricted submarine warfare occurs when submarines attack merchant ships without warning rather than following prize regulations. ... What did the United States do to combat submarine warfare? In WW1, the German policy of unrestricted submarine warfare irritated neutral nations such as the United States and eventually helped public opinion to support the US entry into the war. Entry into World War I, 1917 On April 2, 1917, President Woodrow Wilson went before a joint session of Congress to request a declaration of war against Germany. American Entry into World War I, 1917 On April 2, 1917, President Woodrow Wilson went before a joint session of Congress to request a declaration of war against Germany. 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. In the first six months of 1915, German U-Boats sank almost 750,000 tons of British shipping. In 1917 the use of unrestricted German submarine warfare caused the U.S. to enter the war on the side of the Allies. U.S. Entry into World War I, 1917 On April 2, 1917, President Woodrow Wilson went before a joint session of Congress to request a declaration of war against Germany. In the US mind, they probably felt the same after the Lusitania was torpedoed Unrestricted submarine warfare. Start studying Submarine Warfare in WWI. After war was declared in 1914, Tirpitz urged a policy of unrestricted submarine warfare against the British in the Battle of the Atlantic. Wilson cited Germany's violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, and its attempts to entice Mexico into an alliance against the United States, as his reasons for declaring war. Essay on U.S. Involvement and Impact in WW1; Essay on U.S. Involvement and Impact in WW1 ... events in Europe were altering President Wilson's outlook on the war. A German U-boat had sunk the British passenger ship the Lusitania at great loss of life. How did Germany's use of unrestricted submarines warfare bring the United States into WWI ?