Answer:
Germany's unrestricted submarine warfare and the sinking of the RMS Lusitania were major factors that led to the United States' entry into World War I.
In early 1915, Germany began a policy of unrestricted submarine warfare, which meant that German submarines would attack any ships, including neutral ones, that entered designated war zones around the British Isles. The policy was intended to cut off Britain's supply lines and force it to surrender. However, the policy also resulted in the sinking of many American ships, which outraged the American public and led to calls for the U.S. to take action.
The sinking of the RMS Lusitania was a particularly significant event. The Lusitania was a British passenger liner that was travelling from New York to Liverpool in May 1915. The ship was carrying American passengers and was not armed, but it was carrying munitions and other military supplies. A German U-boat torpedoed the ship, causing it to sink and killing 1,198 people, including 128 Americans.
The sinking of the Lusitania outraged the American public and sparked widespread calls for the U.S. to enter the war. President Woodrow Wilson initially resisted these calls, but the sinking of several more American ships, including the Sussex in 1916, further inflamed public opinion and eventually led Wilson to declare war on Germany in April 1917.
Overall, Germany's unrestricted submarine warfare and the sinking of the Lusitania were major factors that led to the United States' entry into World War I. These events outraged the American public and convinced President Wilson and other leaders that the U.S. needed to take action to protect its interests and its citizens.