Unless the Russians were total psychopaths, their first strike would probably be aimed mostly at our nuclear missiles and the military at large (i.e., a counter

Unless the Russians were total psychopaths, their first strike would probably be aimed mostly at our nuclear missiles and the military at large (i.e., a counterforce strike), and not on purposefully destroying our population (i.e., a countervalue strike). So our nukes would reduce their ability to do second, third, etc. nuclear strikes. It would also destroy their ability to continue to make war, which would prevent an invasion of Europe or the USA after the radiation levels drop. If we had a policy to wait a long time, the Russians could strike our missile bases, large military bases, aircraft carriers, naval bases. But unless we acted fast, our military forces would be in shambles even though the eastern seaboard, Chicago, SOCAL, the major texas cities, etc. are all still standing. We'd still have second strike weapons but they are unusable if we still had domestic civilian targets to worry about. No sane President or General would order a coutervalue strike when faced with a counterforce strike from an enemy. If we let the enemy successfully first strike the United States, Mutual Assured Destruction goes out the window. TL;DR; the Russians could "win" a nuclear war unless we shoot back soon. And if the Russians were total psychopaths who target our cities in a first strike? We have to be prepared to respond in kind or else we incentivize a one sided attack.