How might unrestricted submarine warfare have led to u. s. entry into world war 1

No single event or factor destroyed this neutrality and led to America entering the war on the side of Britain and her allies. American opinion had increasingly turned against the Germans as they stepped up submarine attacks on shipping, including passenger vessels crossing the Atlantic Ocean. The sinking of the British ship, the Lusitania, in May 1915, aroused great anger as 128 Americans were among the 1198 passengers who perished. Further attacks followed until the German government announced in June 1915 that passenger ships would not be sunk without warning.