Why did the United States get involved in World War I?

An excellent
answer, the sinking of shipping and the Zimmerman telegram were the overt reasons America got
into the war.  The Lusitania incident was the most widely publicized, a
little ironic when you consider that the Amercian government and the major newspapers all knew
that the ship had been carrying 200 tons of munitions to Britain, thus making the
Lusitania a legitimate military target under the laws of naval warfare. 
But further shipping losses and the lives of civilian passengers, including both American and
other neutral citizens, led to widespread outrage.

The underlying reason for
eventual American involvement , however, was the same thing that led Britain into the war.  The
British recognized it even before the war began, and President Wilson reluctantly shared the
same view; a German victory over France would leave Europe dominated by a militarist power, and
that simply could not be allowed.  The long-term consequences would have been too
dangerous.