Since that’s unthinkable, a real war is not something worth planning for.
So how about finding a way to win the sort of proxy wars the USA has sucked at since Vietnam (and is currently headed towards for no reason in Iran)?
I'm pretty sure many foreign officials just read it too.
Any real war involving the US vs Russia or China draws in all allies, goes nuclear, and results in the complete annihilation of civilization. Anywhere untouched by direct bombing will be dealing with a radiation cloud which will slowly kill them on top of a global ice-age
Fun reading about that prospect: https://en.wikipedia.org/wiki/On_the_Beach_(novel)
Edit,
Looks like the USA and Russia hold 90% of all the warheads. So a US China war would be a much smaller event. Still pretty devastating to the planet though. 15 to 30 degree temperature drops and a 10 year period to return to normal climate. Plus fallout and famine and EM pulses.
First, if you were (say) France and China launched in the Pacific, would you really doom millions of you’re people by joining in? Really?
Second, NATO obligations are limited to defence, so if the USA launched first (or otherwise started the conflict) other members would be free to opt out. If China hit Japan or Taiwan, NATO would not be involved, the USA would be obliged to get involved (japan is an ally). At that point NATO has no obligation at all and the USA and China are racing to get missiles in the air.
Third, NATO obligations only apply to attacks in Europe or North America (plus French Algeria and a few other “specials”). That’s why the UK fought the falklands solo (often with NATO allies actively undermining it). So there are a lot of US bases that a first strike against wouldn’t actually trigger NATO commitments for. I think (check my geography)than an invasion of Guam wouldn’t be covered. Not sure about Hawaii, but it doesn’t seem to be covered either.
NATO is sort of a mess and since the fall of communism has struggled to find a purpose beyond honouring commitments and organising military activities for preagreed conflicts. But convincing France to sacrifice Paris is a bigger deal than asking for 100 troops for peace keeping in Afghanistan.
I can see the UK following the US blindly. France and other powers much less likely but that's just my opinion.
Let's hope it never happens.
I am actually really glad the Chinese only have ~300 warheads. That's a lot saner than others...
* an unpopular, nationalist president like Trump starts it
* Russia picks up the phone and makes clear it has no intent to attack anywhere in the EU as long as it doesn't get attacked. It offers assurances To Turkey and Eastern Europe and stands down or withdraws troops etc near those regions.
* National leaders (especially say Germany, Italy, Spain) realise their involvement would add nothing and doom all their major cities to annihilation. Also, with the US/Russia gone as powers they will "inherit the earth".
If you were Chancellor of Germany would you really say "fuck it, its worth 50m+ civilian casualties to uphold this point of principle"?
* Russia exerts increased influence on Belarus, turning it into a defacto puppet state
* Russia initiates another grey zone conflict, this time in Eastern Poland, in an attempt to improve the lines of communication between Kaliningrad and the rest of Russia
* While some nations like the UK urge for a diplomatic solution, Poland acts aggressively, striking targets in Belarus to cut off foreign fighters
* Russia denounces Polish aggression and builds up forces in Belarus and Kaliningrad, during which Poland shoots down a Russian MiG that strayed into Polish airspace
* Increased Russian presence leads to incidents along the border. Lithuania begins mobilization, and the other Baltic states soon after
* In retaliation for the MiG shot down, Russia shoots down what it thinks is a Polish F-16, but it actually is Belgian, deployed as a part of the Baltic Air Policing
* NATO forces reinforce Poland and Lithuania, begin to limited airstrikes and special forces raids on Belarussian targets.
* Belarus declares war but Russia officially doesn't, but keeps its troops forward deployed in the country
* NATO forces begin an invasion plan, routing Belarussian forces. Russian troops remain where they are
* Russia enters the war, ostensibly defending their troops stationed abroad. Begins a strike of a variety of strategic targets throughout Europe using ballistic and cruise missiles
* And so on...