[x]

That's what Harry Truman said before he joined FDR's team:

If we see that Germany is winning we ought to help Russia and if Russia is winning we ought to help Germany, and that way let them kill as many as possible, although I don't want to see Hitler victorious under any circumstances. Neither of them thinks anything of their pledged word. -- As quoted in The New York Times (24 June 1941)

But:

1) Once Japan attacked us we were going in on allied side. No more shilly-shallying.

2) Right or wrong, there was a feeling that if we didn't "drain the swamp" this time, we'd have to do it in the next generation. And, as bad as the outcome of the war was for Eastern Europe and some other parts of the world, we didn't have another World War after 1945 (though, to be sure the atomic bomb may have been the main reason for that).

3) You couldn't tell at the time how things were going to end. A lucky break for Germany might have brought Japan, Arab militants, Indian nationalists and Latin American dictators on board and created an even more powerful -- unstoppable -- force.

A Soviet win might have brought them not only Eastern Europe and (eventually and for a time) China, but also Western Europe and Japan. We got involved in the Cold War to prevent that. Possibly our getting involved in WWII had the same effect earlier.

[Tallguy]

(Excerpted) Both AEF commander Gen. Pershing and Allied supreme commander Foch of France were unhappy with the nature of the armistice and subsequent Versailles peace treaty. Pershing believed that it was a grave mistake to let the Germans simply lay down their arms without actually being beaten. (They were defeated, yes, but not beaten.) He correctly predicted that because they did not make the Germans beg for peace on their knees inside a ruined Germany, the Allies would soon be fighting them again. Foch was even more prescient. Upon reading the Versailles treaty in 1919, Foch was heard exclaiming, “This isn’t a peace. It’s a cease-fire for 20 years!” Twenty years and two months later, England and France declared war on Germany.(Snip)

11 Facts about the Great War

If we want to play "Alternate History" imagine what might have happened if Marshal Foch & General Pershing got their way. Germany totally defeated in 1919? The Allied armies on Revolutionary Russia's doorstep, in a position to strangle Lenin's Regime?

I'm reminded of an author who wrote an alternate history of the US Civil War where the Confederacy achieved independence. His methodology was to look at the broad factors, demographics, economics and so forth as more or less 'fixed'. Then he looked for small turning points. Decisive battles being the easiest to identify. Swung those battles on the smallest of events (ie. Pickett's Charge) and looked at how the results of the Campaign might have turned out if the aims were achieved. He identified something like 6 Battles that the Union won, and figured that the Confederacy had to win like 4 of them to achieve Independece. Even then he foresaw an end to Slavery and eventual reunification around the time of WW1 (1912-14).

To defeat the Russians in WW2, Hitler would have had to get on an incredible roll even longer than the one his armies had benefitted from. He'd have had to take Moscow in '41, before the snows. Capture & hold the oil fields around Baku in '42. And somehow avoid the attrition battle around Stalingrad. And that's just for openers.