Vietnam, Afghanistan, and Iraq are different from something like a world war, especially if the United States was attacked first.
Those wars became a matter of "why the hell is the US even there" and segued from a "fight the commies/terrorists" to a matter of "holy crap this is a shitshow, how do we clean this up" that lasted for decades.
In a sense it's much easier to defeat a country with a standing army and a capital city on a map than scattered guerilla forces.
China and Russia aren't immune to social revolution if the population is pushed too far.In the meantime, China, NK and Russia have a history of not giving two shits about approval ratings, dirty fighting and sending soldiers to the meatgrinder. All things the US needs to avoid in order for them to not lose moral support at home.
In fact they're textbook examples of that; that's why they have the governments they currently have. Russia practically flipped government systems three times in the past 100-ish years.
So there's no reason to think that China and Russia's population would "take it on the chin" if their governments play too fast and loose, especially if they see that their efforts are going to waste. They certainly didn't previously.