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.