But they tailor their defense policy in preparation for all of America's might coming down on them, not on whatever fraction we could spare while also taking on three other countries.
Nope the United States could interdict offensive operations by all four powers simultaneously. The US military is not actually built for conquest...something it has proven again recently but it does have tremendous long range destructive capacity without requiring recourse to nuclear weapons.
It lacks perhaps the means to occupy the home territories of the four OP powers with any great degree of effectiveness but only through the possession of nuclear weapons do all of them combined have the power to prevent purely destructive operations. It is worth noting however that even a relatively small number of deployable nuclear warheads of which only a relatively small percentage might reach their targets can inflict many times the damage required to render any military operation politically unpalatable. Then again if any of the OP powers were to risk using their nuclear weapons ( and Iran is yet to gain any capacity and NK's is minuscule and unreliable) they would suffer a counter strike sufficient to destroy any regime.
Hence all four powers work to avoid provoking a US armed response when pursuing their foreign policy.
To engage in a war of aggression you must be able to
1: survive the damage that will be inflicted upon you politically
2: Be able to overcome any potential adversary's efforts to prevent your operations
3: be able to exert effective control over the territories you subsequently acquire.
Currently all powers can effectively checkmate each other at 1 on the defensive because no current leadership can currently present to their people and regime structures a sufficient cause to risk the damage a major conflict would entail. As leaders typically like power and are not prepared to lose it willingly none of them are prepared to authorise such a war as envisaged.
While the US can destroy anyone's (everyone's) military anywhere this only becomes practical outside the territories of each of the nuclear powers as on home soil would constitute an existential threat (literally regimes would fall without their militaries) at which point the use of nuclear weapons becomes a no worse than any other case option. The US could physically survive a nuclear strike by China and perhaps Russia but any leadership would expect to be ousted in the aftermath.
Russia and China could expect to retain a great many physical assets post a US nuclear strike but any regime that invited or even merely suffered such a strike would be most likely dead. If they were not dead then they would have lost the means to continue effective control over their territories.