How can the be a nuclear arms race if everyone already finished the race decades ago and there's enough nuclear bombs to turn the whole world into a wasteland?
It's easy, if you invest in weapons who could potentially stop the other side from delivering the "atomic-payload" you force them to make them "better/different", this is how you start a race without even touching your actual arsenal.
And that's why the US should not place anti icbm stuff near the russian border (turky, poland enc).
Actually that's why the US shouldn't have unilaterally left the ABM treaty in 2002, but by now the Russians are a fair bit along in developing nuclear powered cruise missiles. Who knows, they might be working on (fractional) orbiting nuclear weapons too, they've deployed such systems before.
And that's why the US should not place anti icbm stuff near the russian border (turky, poland enc).