I've always been fascinated by the European assumption that the USA was obligated to help out in any European war, by the way. Especially when we watch those same Europeans not get involved in those same wars till it suits them.
Interesting point. The US entered only halfway WWII. Up to that point they basically did the exact same thing that France, the UK, and Poland did when the Nazis occupied part of Czechoslovakia, and France and the UK did when Poland was invaded: decide that they didn't feel a moral obligation to help the defending side in a conflict that didn't seem to directly concern them.
The people in charge of the UK and France in this period have not been treated kindly by history, even though (to be fair) they probably wouldn't have been able to start a war in 1938 even if they wanted to. Poland would however only be liable to criticism if it had stayed out while its ally France acted, but is not criticized for doing nothing on it's own.
Nowhere you ever see the suggestion that, say, the Netherlands, Belgium, Denmark, of Norway should have acted on such a moral obligation. Some of these have however been criticized for a general lack of preparedness for defending themselves.
The extent of the obligation is clearly proportional to the potential military strength of such a country. Therefore the US as a superpower is nearly always deserving of criticism.
Since the US always had the potential turn the tide in WWII, it's timing and motives deserve to be scrutinized. The US in my opinion did intentionally let the Russians bleed, and perhaps it was right in doing so.
Perception matters here. Most Europeans do not attribute the obligation to police the world to the 25 small to medium-sized countries that form Europe, while increasingly Americans judge the behaviour of "Europe" as a whole and expect more of it.