If it happened a lot would depend on where the fighting was. If China was invaded by the US then China has a HUGE advantage. The same goes for Russia if the US invades from the Pacific Ocean side. If however the main axis of attack is in Europe as was feared during the Cold War, Russia/China would be stonewalled. Anti-tank weapons have never been more lethal and that is generally the Russian attack doctrine(Or at least it was for the soviets). Also air power is key. The West generally speaking, has more and better new generation fighters, bombers and fighter-bombers. The question then becomes one of logistics. Could the Western powers actually maintain supply lines across the ocean and then have enough of an infrastructure after Eastern air attacks to get it to the front lines quickly and effectively. The war would be won on the sea it seems to me, not on land or in the air. And if that is the case...well the West would be in some DEEP shit, because numbers can be negated on land to a certain degree by terrain and supporting air strikes/artillery. Not so much in the ocean.
Side Note: If you haven't read Red Storm Rising by Tom Clancy, give it a look. It's an entirely fictional tale about WW3 and what would have happened if the Cold War had gone "hot". Fairly entertaining as a book, but not sure how credible it is.
Side Note: If you haven't read Red Storm Rising by Tom Clancy, give it a look. It's an entirely fictional tale about WW3 and what would have happened if the Cold War had gone "hot". Fairly entertaining as a book, but not sure how credible it is.