Why was the United States able to play such a significant role in the
Allied victory in world War II?

I am a WWII enthusiast, and am also married to a former citizen of the
Soviet Union.  Many Russians argue that the U.S. played a very small
role in V-E (Victory in Europe), though the contribution to V-J
(Victory over Japan) is undisputed.  The U.S. entered WWII after the
USSR had already begun to push the Nazi's back from Moscow, and
Russian's percieved that victory was within their grasp.  It is
possible that the U.S. entered the war to prevent a communist takeover
of Europe.

The U.S. was able to play such a large role for several reasons.  One,
it was never directly attacked in any significant manner, the
factories and farms in America were at full production year-round. 
Two, while recovering from the depression of the 1930's (following the
stock market crash in '29) the contracts put out by the government to
built planes, jeeps, ships and guns revived the economy.  Three, the
European powers, especially Britain, invested heavily in American
supplies of food, gunpowder, bombs, and aircraft.  This extra funding
also provided a boost to the U.S. economy.  Perhaps the most important
reason was the eagerness of everyone not just to work, but to work
hard after a decade of 25% unemployment.

The most critical contributions made by USA in WWII were:

1: Helping Britain to survive 1940 by providing destroyers for bases;
patrolling the high seas, etc. The 'Undeclared War'.

2: Arranging the funding of the war through 'Lend-lease' facilities in
March 1941, actually a huge War Loan that eventually totalled some $50
billion.

3: Becoming the 'Arsenal of Democracy' by manufacturing war materiel
in facilities that were out-of-range of German bombers.

4: Providing Intelligence collection and decryption resources that
complemented those of Britain.

5: By collaborating in the North African campaign and the invasions of
Italy and then France that led to the defeat of Germany in May 1945.

6: By devloping and ultimately employing Nuclear Weapons in 1945 to
enforce peace with Japan in September 1945.

And why was it able to play such significant roles?

Sheer economic clout combined with a relatively liberal regime that
abhorred the excesses of the Nazi regime, aided by great propaganda.

For me, the role the US played in WW2 has been a topic of unsettleable
dispute for many years. The 'juggernaut' of US participation has been
said by some to be overblown. Some say that the European powers would
have won without the help of America. But I can agree that US
partipation made the war last much shorter than if they had not
participated. In a way, the US was climbing to the top in those days,
but WW2 participation gave the ultimate boost for it to become the
most powerful nation on Earth.

Sorry, Techtor ...

But I disagree.

Without American support & Lend-Lease, Britain would have settled for
a Peace Deal with Hitler in 1940 or 1941. That's for sure.

Hitler would have then be free to invade Russia without worrying about
having to protect his rear. Who knows what then might have happened?
Possibly Japan may have entered the war against Russia. That was the
idea of the Tripartite Anti-Comintern Pact.

Don't forget that Russia was also getting Lend-Lease supplies and when
the USA went to war against Japan it took Japan's interest away from
Russia.

There were A LOT of countries undecided which way to vote and Hitler
was the early favourite.

Had he been allowed to continue, the Middle East was his next step and
whoosh all the oil business would have fallen into his lap.

As an Englishman I don't think that we can ever thank Uncle Sam & his
Fifth Cavalry enough for riding to our rescue at the last minute.

It was a near run thing!

The more I read about Hitler & his Thugs, the happier I am that he was
stopped in his tracks.

Can?t presume to add to the factual stuff presented by others, but to
interpret the question in a slightly different way -- why was the USA
*able* to play such a significant role. Is this what you meant?
Without the Pearl Harbour attack, US politicians couldn?t have swung
the voters to overt involvement. The US seem to take attacks very
personally.

Best

Probonopublico,
Really, I didn't think you as a Englishman would think highly of the
American participation in World War 2! I had assumed the UK didn't
like Uncle Sam's taking center stage in The Big War. But I wouldn't
disagree with you either on that, the US truly had a big role in
quelling the threats of the time. That the threats would not have been
quelled at all if not for the US coming, perhaps you are right. That
the UK would have had a truce with Hitler, really, that enlightened
me! Although there could be more to history than that... Actually, I
admit being biased in a way for the US, being staunchly pro-American
as a kid (because of repeated episodes of Vic Morrow's Combat when I
was 5 years old), and as a scale modeler, I'm pretty fond of American
warbirds. :)

If it is possible I would like to have an answer for this question.
the comments that I have received are very interesting, but I feel
that they are too personal opinions, and I need an answer based on
facts. thanks. lokita-ga.