What factors led to the Allied victory in World War I?

WWI was won by the Allies mainly by the fact that the US, with
its huge population and industrial capacity, entered the war.

Up
until the US entered the war, the war had been a real stalemate. The Western Front, where most of
the important fighting was going on, had not moved any significant distance in years. This showed
that the Allies and the Germans were essentially deadlocked.

When
the US entered the war, this all changed. The US had the potential to send millions of men and
huge amounts of materiel into the war. This convinced Germany that it could not
win.

The US entered the war due to its economic and political/social
ties to England. The US conducted huge amounts of trade with the British and had deep historical
ties to Britain. This, combined with the German submarine warfare, brought the US into the
war.

So, the Allies won because they were able to get the US on
their side. The economic and political/social ties between the US and Britain (and to some degree
France) brought the US into the war, changing the strategic balance.