Germany created a new submarine called the "U-Boat". Germany gave a "sink on sight" order to any ship approaching Britain because the British blockaded the North Sea. On May 7th, 1915, the British passenger liner the "Lusitania" sunk off the coast of Ireland killing 128 Americans. Then, in August 1915, the Arabic was sunk by the Germans and 2 Americans were killed. Lastly, in March 1916, The Sussex, an unarmed passenger ship was sunk. Due to all this, America threatened to cut off diplomatic relations. The Sussex pledge stated that no passenger or merchant ships would be sunk without warning.
Unrestricted Submarine Warfare
The British made the American public feel bad for them. They send news of the war full of propaganda every day.