What was the situation when the U.S came into WWII, and how did the Allies win?

World War II was a long drawn out war, fought between
countries coalition of more than 50 countries including the most powerful countries of
that time, and extending over three major continents. In a war like this it is not
possible to speak of just one unchanging foreign policy. In a dynamic situation like a
world war is necessary to have a flexible foreign policy in line with changing
situation.

For example, initially the policy of USA was to
not to participate in the war only selling war material for cash to allied country. This
was changed to the policy of supply on lend-lease basis. Subsequently USA decided to to
participate in the war directly.

Similarly take the case of
Russia. Initially it did not oppose the actions of Germany, but was forced in to the war
nearly one and half year after the start of war. In situation like this foreign policy
of any country toward Russia could not have remained same throughout the
war.

Similarly, Bombing of Pearl Harbour by Japan in
December 1941, made a shift in foreign policies of all the countries, towards Japan in
particular and towards the war in general, absolutely
essential.

Coming to the question of how Allies won the
war, to me the most important factor appears to be the greater combined military and
economic strength of the allied countries.