In 1915, the British Royal Navy’s blockade of Germany began to have an effect. In response, the Germans attempted to blockade the British Isles with submarines. Previously, German U Boats would surface, stop a target ship, board it, search it, and if it was carrying war materials, allow the crew and passengers to abandon ship. This was known as “Prize Rules” or “Cruiser Rules”. However in the spring of 1915, Germany dropped the traditional Prize Rules, and began unrestricted submarine warfare. U Boat captains no longer had to warn or search their targets beforehand. They could just approach stealthily and fore a torpedo. Furthermore, neutral ships were no longer off limits. Any ships near the British Isles were fair game.
In early May 1915, the passenger liner RMS Lusitania was on her way from New York to Liverpool with 1,952 civilian passengers on board, including 197 Americans, and was secretly transporting munitions for the British war effort. However, her captain, William Turner, did not believe the U Boat threat was serious. The Lusitania didn’t need any escorts because she was fast enough to outrun any U boats, but Turner had only three of his four boiler rooms working to save coal which reduced his speed. He also didn’t zig zag because he felt it was a waste of time. Finally, he neither checked reports of recent U Boat activity in his area, nor avoided their traditional hunting grounds. The stage was set for a disaster.
On the morning of 7 May, U-20 commanded by Kapitanleutnant Walter Schweiger spotted the Lusitania off of the coast of Ireland. He fired a single torpedo which struck the Lusitania’s starboard side. The Lusitania sank in just eighteen minutes and 1198 passengers died, including 128 Americans. It was the second largest loss of life at sea up to that time, behind only the Titanic which struck an iceberg three years before.
In 1915, America was not participating in the Great War and had no plans to. The prevailing mood among Americans was that this war was no different than the many small continental wars that occurred over the last 50 years in Europe, like the Franco-Prussian War of 1870. Furthermore, the largest immigrant group in America at the time was German, and although slim, there was still the possibility that America would join the Great War on the side of Germany. More likely though, their immense political pull would keep America out of the war altogether. They left Germany to escape the continental feuding not get involved in it. The sinking of the Lusitania changed all of that.
International opinion, particularly American opinion, turned irrevocably against Germany after the Lusitania was sunk. Unrestricted submarine warfare kept the Great War on the front pages of American newspapers. As long as Germany continued unrestricted submarine warfare, it was not a matter of if America would join the war, but when.