When did the US enter WW1?

Although President Woodrow Wilson had pledged to keep America out of the war during his presidential campaign in 1916, he quickly changed his mind and asked Congress for a declaration of war on April 2, 1917. Both the House of Representatives and the Senate unanimously agreed with Wilson, and the United States promptly entered the war on April 6.
There were several reasons for Wilson's change of heart. First, Germany had resumed unrestricted submarine warfare after promising the United States, in the "Sussex Pledge," not to attack non-combatant merchant vessels without warning. Second, the Zimmerman Telegram, intercepted by the British, indicated that Germany was aligning with Mexico in the event of American involvement in the war. Third, public opinion had been swayed by stories of the atrocities committed during Germany's invasion of Belgium. While many of the stories were simply British propaganda, they whipped up anti-German sentiment in the United States. Another (less recorded) reason for the war was economic. By 1917 American banks had loaned Britain and France $2.25 billion and risked losing that money in the event of a German victory.
American troops led by General John J. Pershing reached Europe in June 1917 but only saw limited action that year. Full participation by American troops didn't occur until the summer of 1918 at the battles of Chateau-Thierry, Belleau Wood, and Saint Mihiel, all allied victories. Most historians agree that the weight of the American forces, as well as the naval blockade of the German homeland, caused Germany's ultimate defeat.