As I have been summoned!
Barring a pre-Cold War PoD or a borderline ASB scenario like, I don't know, the USSR suffers a "brokenback war" where it is the only one with the brokenback while the US is killed dead, an unconditional surrender is never going to happen (and even then that isn't so much an unconditional surrender as "the other guy's dead and we'll be joining them in five years or so" type of victory). A conditional surrender is best engineered by the eventuality of a political failing on the behalf of the Western Allies. That requires some kind of start to the conflict which fails to produce a rallying-around-the-flag event, either in a domestic sense (ie: the US public splits badly enough to torpedo the war effort and force a Soviet-favorable peace deal) or a international sense (ie: the NATO alliance splits badly enough to torpedo the war effort and force a Soviet-favorable peace deal). That's... a pretty difficult sort of conflict starter to find, especially for democratic nations where policy decision-making is relatively more diffuse then in a oligarchy or autocracy.