The fortunate thing for the United States, if it came to WW3, is that it's got an ocean on either side. If China or Russia would want to invade the US, it would have to send it's army either across the breadth of the Pacific, and then it would basically be a repeat of the Pacific theater in WW2, or it could do things the "Fallout" way and invade Alaska. Alaska's vulernable to both, but it's not at all heavily populated, and then the invading army would have to either run a bloody gauntlet shipping troops down the coast, which would leave them vulenerable to sea, air and ground attack, or they could try to traipse through the vast northern Canadian wilderness to try and attack the Great Lakes, which would be terribly ironic since the Russians would be facing the same problem that they've given all of their invaders. Either way, they would have a very tough time with a full land invasion.
A Russian invasion of Western Europe, of course, would play out as a high-tech version of 1943-1945, even if you discount nuclear weapons. A Russian invasion army could definitely be targeted by a tactical nuclear strike or ten, and then it would lead to all-out nuclear war. That is why every nation's been fighting proxy wars since 1945. Any nuclear nation fighting another will just have the prospect of warheads dropping. And, really, no one wants Mutually Assured Destruction.