They did have German advisors training and fighting with the Chinese
Most of the Chinese soldiers were relatively useless and more concerned with killing Chinese soldiers than fighting the Japanese, but with the exception of a few prominent warlords who relentlessly drilled their soldiers, the German-led armies were pretty ard as nails
STOP IMPLYING MY HISTORY-FU IS WEAK.
I know that Nazis helped China, the point was that even after Germany allied with Japan and ceased all support to China, they never, ever, actually took hostile action. Sure, yeah, China as part of Allies declared war on the Axis, but there was no fighting between Germany and China specifically.
That would make sense, but still doesn't quite cover the air superiority of the USA
Russia might have enough air dakka but cracking fortress USA is ard
It is, which is why I said it would proably devolve into some kind of modern attrition war (provided noone uses nukes, in which case it's a moot point anyway) where it would boil down to noone getting significant advantage, albeit with constant attacks on rear (special forces, terrorists, Tomahawks and shit) which would finally end up in complete isolation of America, Big Brother-esque internal security. Ultimately, it would result in USA loss, since the rest of world simply has more land, which means more resources and also more people.
Also, a lot depends on how far we're willing to go. Nukes are out in any scenario that implies actual combat, but what about biological and chemical weapons? Russia invested (and supposedly invests) a lot into that and it would significantly impact the war - sure, the troops have NBC gear (but gas mask filters will wear out eventually) but majority of civilian population does not have any real way to protect themselves.