Originally, the United States hoped to stay out of the war; in fact many considered the Atlantic Ocean as a comfortable buffer to prevent that from happening. Literary Digest printed an article which read in part:
Our isolated position and freedom from entangling alliances inspire our press with cheering assurance that we are in no peril of being drawn into the European Quarrel.
A series of events over time led to U.S. entry, the first of which was the sinking of RMS Lusitania on May 7, 1915 by a German submarine which caused the death of 128 Americans. President Woodrow Wilson strongly protested this event, even though there is evidence that the Lusitania was carrying weapons and would thereby be a fair target. In an attempt to prevent U.S. entry into the War, the German government issued the Sussex Pledge:
Liners will not be sunk by our submarines without a warning and without the safety of the lives of non-combatants provided that the liners do not try to escape of offer resistance.
Events thereafter slowly moved the U.S. toward war. The British cut the cable between the U.S. and mainland Europe so that all news of the war reaching America was from Britain. President Wilson called for "Peace without victory" in a speech before Congress, but events had taken a different turn.
The German government, stung by the British blockade and unable to enforce its own blockade of the British isles, sent a diplomatic note to President Wilson on January 31, 1917 stating they would resume unrestricted submarine warfare (sinking ships without warning) the following day. On February 3, Wilson broke diplomatic relations with Germany.
Then, on February 25, 1917, the British delivered a decoded German telegram addressed to the German Embassy in Mexico City asking Mexico to join in the war against the U.S. in exchange for which Mexico would regain the "lost territory" which the U.S. had gained from Mexico in the Mexican War of 1848.
On March 17, 1917, German submarines sank three American ships. Wilson then asked Congress for a formal declaration of war on April 2; and war was declared on April 6, which ironically was "Good Friday."
The main event was the sinking of the British passenger liner, the Lusitainia, in 1915 that killed 128 Americans. Another reason was the letter intercepted from Germany to Mexico stating that Germany would aid Mexico in invading the U.S.