Originally, with the support of President Woodrow Wilson, the United States remained neutral in World War One.
We must remain neutral!
President Woodrow Wilson
But when the German's started torpedoing and sinking U.S. ships, most famously the Lusitania....
And when the United States intercepted a telegram, sent January 19th, 1917, (known as the Zimmerman Telegram) from the leader of Germany, Arthur Zimmerman, to the leader of Mexico, Heinrich von Eckardt, saying that if Mexico joined the Axis Powers, Germany would help them take over America (even though Mexico didn't accept the offer)....
Among other reasons, Congress (now with the support of Woodrow Wilson) voted to enter World War One on April 6th, 1917......
We declare war!!!
Speaker of the House Champ Clark
With the Senate voting 82 supporters of entering the war, and 6 supporters of staying out of it and with the House of Representatives voting 373 supporters of entering the war, and 50 supporters of staying out of it.