It was a series of things over a number of years, the main ones being:

1. The sinking of the Lusitania, with 128 Americans on board. It was a British ship with munitions on board, so the Germans had reason to target it, but it was still a civilian ocean liner with all that implies.

2. Germany's policy of unrestricted submarine warfare. Germany was being strangled by an Allied naval blockade and the Germans decided their only hope was to use their U-boat fleet to impose an absolute blockade on shipping to Britain and France. This meant that all ships bound for Britain and France, even if they were American or civilian, would be sunk on sight by German U-boats.

3. The Zimmermann telegram. Fearing the U.S. might enter the war on the side of the Allies, Germany offered Mexico the chance to join the Central Powers. The proposed deal was that Germany would provide the funding for Mexico to go to war with the U.S. and regain the territories which it had lost in the Mexican-American War back in the 1840s. Mexico turned Germany down flat, but the U.S. found out and got angry.

What were the U.S. goals? Well, President Woodrow Wilson had developed a vision of remaking the Europe of monarchical empires into a Europe of democratic nation-states. Wilson's hope was that if America played a decisive part in winning the war, he would have enough influence over the peace process to put his vision into reality. He got his wish for the most part, but his hope that this would bring about everlasting peace didn't work out so well.