Why did the US enter World War I?

The primary events that led to the United States
declaration of war against Germany were the Zimmerman Telegram and Germany's announced
intention to resume unrestricted submarine
warfare.

American sentiment had leaned toward the Allies
and against the Central powers for some time. Americans felt a common affinity toward
the British because of the common language. Also Woodrow Wilson made no attempt to hide
his disdain for persons of German ancestry. He once referred to German Americans as
"hyphenated Americans."  Sentiment against the Germans was also intensified after the
sinking of RMS Lusitania in 1915; however this was NOT the cause of
U.S. entry into the war. After that event President Wilson famously commented "there is
such a thing as being too proud to fight."

Following the
sinking of the Lusitania Germany had issued its Arabic
Pledge (sometimes known as the Sussex Pledge) in which
the High Command promised it would no longer sink Allied ships without first giving
appropriate warning. Several things changed this: The British often few the flags of
neutral countries on their ships, and also rammed German U-boats while pretending to
allow boarding. Then too the war was at a stalemate, and the Germans had to do something
to move the war effort. This led to their notice on January 31, 1917 that Germany would
resume unrestricted submarine warfare the next day.

On
February 25, 1917, President Wilson received an intercepted German telegram to the
German Embassy in Mexico City that offered to Mexico the "lost territory" of Arizona,
New Mexico, etc. (land lost by Mexico to the U.S. in the Mexican American War of 1848)
if Mexico would declare war on the U.S. The British had intercepted the telegram, and
delivered it to the U.S. in an obvious attempt to secure U.S. entry into the war on the
Allied side. On March 17, 1917, Germany sank five U.S. merchant vessels, and President
Wilson delivered a war message to Congress on April 2. War was declared on April 6,
1917.