The U.S. was drawn into World War I for a multitude of reasons. Three of the most important are as follows;
1) The Invasion of Belgium: At the beginning of the war, German decided that if they attacked France by going north through Belgium it would save them times and lives. The invasion didn’t go as smoothly as anticipated when the Belgium people began resisting, and soon rumors of German atrocities, many of them highly exaggerated, made their way to the U.S. and angered he public.
2) The Sinking of the Lusitania: Germany was having only marginal success stopping the flow of supplies to the Allied powers during the war. Ammunition and other war materials were being smuggled aboard American passenger ships, so when the German government changed it policy regarding submarine warfare, they began targeting passenger liners as well. One ship in particular, the Lusitania, was torpedoed in 1916 while carrying several American passengers onboard. The response in the U.S. was huge, and soon people began demanding that the government avenge the dead aboard the Lusitania.
3) The Zimmerman Telegram: In an effort to make the U.S. think twice about deploying a large force in Europe, the German foreign ministry sent a note to the Mexican government hinting that if Mexico were to declare war on the U.S. in the event the U.S. went to war, then Germany would support them. Mexico was still bitter from the loss of California and the rest of their northern territories, but the deal didn’t resonate with the Mexican government. Unfortunately for Germany, the U.S. intercepted it and the public outcry for war grew even louder.