What was the incident on the seas that angered many Americans in 1915?

On May 7, 1915, a British passenger ship carrying 128 Americans and 4200 cases of ammunition was sunk by a German U-boat. American lives were lost. This incident infuriated many Americans who now called for war. President Wilson sent a pair of notes to Germany, asking for reparitions and a promise that passenger ships would not be sunk. In February 1916, Germany apologized. THE Lusitania