No doubt we can take all of those countries at once, but if you factor in logistics, coordination between our allies, amassing troops to match the scale of the conflict(China has by far the most troops), positioning, preparation, etc., it would take time to get ready for a big conflict of that magnitude. Hell, if Russia poured all of their troops into Ukraine, NATO would be already at a disadvantage since it would take time for them to transport all of their equipment and troops across multiple countries. Militaries can't just magically appear in an area ready to act on a whim. You have to account for multiple battle groups traveling in different speeds to position themselves at multiple strategic points to carry out an offensive, and that's not even factoring in the units already in place that have to deal with the enemy until the main force arrives. So essentially, if we were going to send in our troops from east Germany to take on Russia, they would have a big numerical advantage in EVERY UNIT until more battle groups get there. And by the time that happens, China and Kazakhstan would already be waiting for us dug in with North Korea sending more troops while more of ours are coming from western Europe. On paper, we are the superior military tactically, technologically, effectively, intelligently, etc. But in reality, variables can play a big part in taking advantage of battlefields(taking up high point positions, securing towns/cities, setting up supply lines, knowing the environment, intelligence in the ground, air superiority, setting up defensive positions, etc.) It would be a drawn out ugly conflict that would cost countless amount of lives and it will all stem from a conflict that never necessarily required our direct intervention. We could absolutely carry it out, but we'd be playing catch up until for a while until we could transport everything we needed to the battlefield. The US doesn't carry out wars like that, we usually overwhelm the enemy and use "Shock and awe" to confuse our them while cutting of communication, surveillance, and supplies keeping them unbalanced, blind, and disorganized(see Desert Storm).
