Final answer:
The United States entered World War I in 1917.
Step-by-step explanation:
The United States entered World War I in 1917. Prior to that, the United States maintained a policy of neutrality. However, several factors led to the US joining the war, including the sinking of the passenger ship Lusitania by a German submarine in 1915, the interception of the Zimmermann Telegram in 1917, and the resumption of unrestricted submarine warfare by Germany. These events eventually prompted the US to declare war on Germany and its allies in April 1917.
Learn more about US entry into World War I