Final answer:
The sinking of the RMS Lusitania by a German U-boat in 1915 and subsequent unrestricted submarine warfare against American vessels shifted U.S. public opinion and were significant factors in the U.S. decision to enter WWI by 1917.
Step-by-step explanation:
The sinking of the RMS Lusitania played a significant role in propelling the United States into World War I. On May 7, 1915, the Lusitania, a British passenger ship, was sunk by a German U-boat off the coast of Ireland, resulting in the loss of 1,200 lives, including 128 Americans. This event was a catalyst for changing American public opinion because it was widely seen as an act of unwarranted aggression. Despite claims of self-defense by Germany, who pointed out that the ship was carrying military supplies, the attack on a civilian vessel caused outrage in the U.S. Subsequent sinking of American vessels by German U-boats further increased pressure on President Wilson, who eventually led the U.S. to declare war on Germany in April of 1917.
Moreover, the German naval policy of unrestricted submarine warfare was seen by many in the U.S. as a challenge to neutral rights and a threat to American citizens. Shortly before the Lusitania's sinking, the German Embassy had issued warnings regarding travel on British ships. Nevertheless, the actual sinking significantly influenced the public and political sentiment, contributing to growing support for American intervention in the conflict. With the expansion of the U.S. military by the National Defense Act of 1916 and increased tensions caused by further German U-boat aggressions, American involvement in WWI became increasingly inevitable.