The landscape of war and its very definition has changed over the last 50 years. Cold wars, civil wars, wars on drugs and poverty, cyber warfare, war on terror and so on have caused a shift in the reality of what "war" consists of.
Having said that, I think it's very unlikely the war between the US and China or Russia will be an armed conflict, Not when there are SO many other ways we can hurt each other now.
It is amazing how some armchair soldiers and even out-of-touch veterans still have not come to grips with what you said.
In the age of nuclear weapons, mass invasions, nationwide carpet bombing, long-range missile attacks, etc., which have been and currently are the strengths, of the US imperial military, are obsolete relics of WWII-type warfare and out-the-question, between nuclear arms peers. China and Russia especially know this, as it explains why they have not tried to establish imperial garrisons all over the planet, as does the US empire. Those far-away outposts, themselves, are not only endless money-pits, but are also obsolete imperial fixtures, given the capabilities, of tactical nukes.
I would even venture to offer that there are a number of people on this forum, who could easily devise an effective strategy to defeat the US empire without heavy reliance on the traditional military. Full-spectrum cyberwarfare, crashing the US dollar and/or moving to another currency-of-trade, and the creation of new economic blocks or exclusive trade zones, on a global basis, are but a few 21st weapons of war that can effortlessly sidestep the military might, of the US empire.
However, as said numerous times before, China and Russia does not even need to do any of the above, as the the US empire itself, is rapidly declining from within. The only real mission the militaries, of both China and Russia, is to prepare for the geopolitical vacuum, after the collapse, of the US empire; which could suddenly happen much sooner than its subject citizens realize.