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.