At first the United States tried to stay neutral, and by 1917, Woodrow Wilson created a policy about neutrality in the War and he announced that the U.S has to be neutral.
One reason the U.S decided to enter, however, was because of the horrible doings of the Germans in Belgium, and the sinking of the Lusitania (a German submarine sunk the Lusitania filled with many English and American citizens) which is most popularly viewed as the reason for the United States getting involved.
