Right, it does fall apart if you look at it that way- the soviets being dead doesn't make Americans less dead. It does make sense if you open it up to a wider timescale, though- if the Soviets knew you'd retaliate because you'd been doing that all along about the smaller things, they'd be less likely to launch a nuclear first strike. The principle of mutually assured destruction is a strong deterrent.

Certainly many on the left won't have enough self-awareness to think about it that way, but unilateral disarmament is indistinguishable from surrender.