Why did the US enter WWI and what effect did its entry have?

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.