Why did the US enter World War I?

There are two
main factors that led the United States to enterafter having stayed neutral for so
long.

First, there was the fact that the Germans had resumed unlimited
submarine warfare.  This had been a point of contention between Germany and the US for most of
the war.  The Germans had stopped using the tactic for a time, but started up again in January
of 1917.  A number of American ships were sunk in March of that year, leading to the declaration
of war in April.   The US felt that this type of warfare was illegal and inhumane.  The
government also did not like the fact that it reduced American trade with Britain.

The other immediate cause was the Zimmermann Telegram, which tried to persuade Mexico
to enter the war so as to keep the US tied down at home and unable to help the Allies in
Europe.  This angered many Americans.  Americans were particularly angered by the fact that the
Germans offered to give Mexico land that the US had taken from it in the Mexican-American
War. 

These two factors led to the end of US neutrality in this
war.