I think the current arms race started in 2002 when the US pulled out of the ABM, followed by the "upgrades" which started under Obama. (The research for those upgrades of course started long before - it's the deployment I'm interested in.)
I think adding super-fuze to US nuclear weapons - "In the case of the 100-kt Trident II warhead, the super fuze triples the killing power of the nuclear force it has been applied to. https://thebulletin.org/how-us-nuclear-force-modernization-u... - is an example of not just of the "mere existence of a nuclear arsenal", but a determined effort to tilt the balance very much in favor of the US.
Why did we need that upgrade?
I think the US has developed its systems far enough in that direction that Russia is now forced to spend money to keep up, as otherwise MAD is no longer effective.
And that's why I call it an re-uptake of the arms race.
What would you expect to see if there were a new nuclear arms race?