The main factor that led to the decision of the U.S. to enter World War I was the unrestricted submarine warfare conducted by Germany. This policy, particularly evident in incidents like the sinking of the RMS Lusitania in 1915, resulted in the deaths of American citizens and heightened tensions between the U.S. and Germany. Additionally, the revelation of the Zimmermann Telegram in early 1917, in which Germany proposed a military alliance with Mexico against the U.S., further swayed public opinion. Together, these provocations, alongside economic ties to the Allies, prompted President Woodrow Wilson to request a declaration of war in April 1917.