On May 6, 1915, the RMS Lusitania was torpedoed by the German U-boat U-20, just 11.5 miles off the coast of Ireland, sinking in 18 minutes. Of the 1198 passengers who died, almost 100 were children, and she also carried 139 Americans, 128 of which died in the sinking. Among the passengers were many American and British dignitaries and luminaries of the day.
The resulting world public backlash pressured the German leadership to revert to a policy of Restricted Submarine Warfare. They reverted back to Unrestricted Submarine Warfare (URSW) in 1917 to counter the British blockade of Germany, hoping to break the back of British shipping and to delay the U.S. entry into the war, but by then it was too late. Had they held fast and continued their policy of URSW after the Lusitania, WWI may have turned out much differently.
Their reversion back to a URSW policy was also one of the key elements leading to the entry of the United States into WWI.
The renewal of unrestricted German submarine warfare had devastating effects on shipping in 1917. In terms of tonnage, the Germans sank 540,000 tons in February 1917, 578,000 in March and 874,000 tons in April. Anti-submarine measures by the Allies, however, began to reduce this calamity.
submarine
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Germany resumed unrestricted submarine warfare in 1915. The Germans declared the waters around the British Isle a war zone. The would attack any merchant ship entering that area.
the Germans' use of unrestricted submarine warfare
Primarily by submarine warfare. The materials sent for lend-lease went by ship.
Enemy surface ships are the main target of submarine warfare
Unrestricted submarine warfare is a naval war tactic in which the submarine attacks merchant ship (or any ship not in the navy) without warning. Germany used this tactic on American vessels at the start of WW1, and that is a major reason that the US joined the war effort against the Germans.
Germany hoped that submarine warfare would lead to war.
Anti submarine warfare
Germany's submarine warfare was unrestricted. These attacks led to World War I.
When the Germans, declared unrestricted submarine warfare, the US issued a sharp protest to this policy, saying that it was an indefensible violation of neutral rights, and threatened to hold Germany accountable. After many German offenses in the seas, Wilson threatened to break off diplomatic relations with Germany unless it abandoned its methods of submarine warfare. Wilson won a temporary victory when the Germans said that they would visit all vessels prior to attack. However, this crisis prompted Wilson to begin preparing for war (preparedness campaign). Finally when the Germans decided to declared unlimited submarine warfare on all shipping in 1917, it was impossible for Wilson to maintain US neutrality and the freedom of the seas. Thus, Wilson broke off all diplomatic relations with Germany and called on Congress to approve the arming of US merchant ships.