The stated reason for the entry of the United States into the First World War was to protect democratic nations from the threat of anti- democratic ones.  The perceived attack from Germany helped to justify the need for entry into the war.  On another level, others argue that the United States was waiting for a reason to enter the conflict, that there is enough historical research to indicate that the Lusitania incident could have been preempted a bit better than it was and that America knew of sentiments percolating before it.  Regardless, the United States recognized that while Europe was mired in a brutal war for control, it could not remain neutral for very long.  Its entry into the conflict clearly turned the tide in favor of the Allied powers and rendered the Axis into a position of defeat.

The US had supplied weapons to the Allies exclusively during the war, to the tune of several billion dollars worth.  The German sinking of the Lusitania in 1915 was largely due to the fact that German spies had reported there were arms shipments on board. So one has to say that American support of the Allied Powers with weapons shipments was one reason why we eventually entered the war.

I think the situation in 1917 was actually closer than the above post suggests, which was one of the factors leading the US into war.

The Russian Revolution in 1917 had given the Germans victory on the Eastern Front with the Treaty of Brest-Litovsk, allowing the Kaiser to concentrate his entire army on the Western Front, which had stalemated into bloody trench warfare with both side bled white.  A final offensive by a reinforced German army might capture Paris and end the war for good.  This was a real possibility when the US came in on the Allied side.  In fact, one of the major effects of our entry into the war not yet discussed here was the American Army's role in breaking up that much of that last offensive at Chateau Thierry and Belleau Wood.

The US had supplied weapons to the Allies exclusively during the war, to the tune of several billion dollars worth.  The German sinking of the Lusitania in 1915 was largely due to the fact that German spies had reported there were arms shipments on board. So one has to say that American support of the Allied Powers with weapons shipments was one reason why we eventually entered the war.

The second to last link below refers to Chateau Thierry, which was also called the Second Battle of the Marne.

There are a number of arguments about why they eventually joined the effort including an allegedly (shown after the fact) fake telegram that was supposed to indicate some plot by Mexico to try to take back territory in the Southwest should the US enter the war, etc.  There were incidents like the sinking of the Lusitania that turned American opinion closer to wanting to go to war and it was the...