“Deterrence of the Soviets never depended on having “rational” leaders. Stalin was in charge when the Soviets first began a build-up of nuclear arms, and it is difficult to consider him as an example of a rational leader.”

A very questionable statement. Stain was the most rational leader of the 20th century. I offer as evidence his longevity as the ruler of the Largest nation of the 20th century.
Anyone that doubts how rational Stalin was is using the wrong metric.

“But with Russian conventional forces vastly inferior to those of the US, the Kremlin’s nuclear leftovers formed the only means of sending Eastern Europe the proverbial severed finger.”
As Eastern Europe’s acceptance of the Missile is a way of sending Russia the lifted finger.
Plus PRESIDENT Bush scared the crap out of a bunch of despots when Saddam was dropped from the gallows. Putin was just one of many who could visualize his own self swinging from a gallows.
America is ENORMOUSLY POWERFUL here at the dawn of the 21st century. It’s not just the military part, but the economic and cultural advantages are perhaps even more intimidating. You cannot defeat America at war and yet if you don’t, you will be assimilated ( globalization ) eventually.
To see a President attempt to actually use that power, no matter how clumsy he was, had to strike fear in the hearts of Despots everywhere.
The Lead the US has in the Military arts to so large, that if we stopped working now, it would take over a decade for anyone else to catch up. And the pace of American military development is accelerating.
I assure you, the thought of a compenent American President causes Putin and the other despots to wake up in the middle of the night in a cold sweat.
Fortunately for them the American political system is designed to produce incompetent Presidents and prosper anyway. Another safeguard against Tyranny.