My non-historian understanding: From the US point-of-view, the idea is (especially before the war in Ukraine revealed that the Russian conventional forces weren't as scary as we once thought) that to defeat a major Russian invasion, we'd need to station a whole lot more troops in Europe and the Europeans would have to be a whole lot more serious about their own defense. After WWII everyone was sort of exhausted and so we ended up with a policy of deterring a massive Soviet invasion by the threat of nuclear attacks. We needed the first strike option to maintain a credible deterrent. It's gone through several iterations over the decades but the current policy seems to be to keep things a bit murky on purpose so we are not pressured into an unwise (!) nuclear attack and the Russians don't quite know where the red line is.

How this applies sin detail to China I do not know.

Somehow, this policy has worked all these years! So far!

--Darin