The primary factor in America's decision to enter World War Iwas Germany's resumption of submarine attacks on American vessels, although the Zimmerman Telegram and British war propaganda also had an impact. Although the United States had initially attempted to stay neutral in the conflict, public sympathy slowly shifted toward the Allies, leading to President Woodrow Wilson asking Congress to declare war on April 2, 1917.
Initial resistance to the war was strong. Great Britain's refusal of Irish independence turned most Irish-Americans against them, while many German-Americans feared they would face persecution if America went to war against Germany, which turned out to be prescient. The Midwest in particular was strongly isolationist.
From the beginning of hostilities, Britain tried to persuade America to join the Allies. German atrocities committed against civilians in Belgium helped their cause, as did Germany's bombing of London and use of new weapons like mustard gas. British intelligence also leaked the Zimmerman Telegram in February, 1917; it detailed an attempt by the German government to enlist Mexican aid against America in exchange for territory Mexico had lost during the Mexican-American War.
The tipping point, however, was the previously mentioned resumption of U-boat attacks on civilian ships. Germany had already outraged the general U.S. populace previously with the sinking of the RMS Lusitania, a passenger liner that included 128 Americans among its 1,198 passengers when it was torpedoed in May 7, 1915. When Germany announced that it would attack American ships in response to ongoing material support for the Allies by the United States and subsequently sank five ships, public support for war was finally strong enough to move the government to action.