Tacnuke

Probably it would go as this: the Soviets attack with as little warning as possible in a holiday with a massive BC assault together with their mechanized wave. Maybe a tacnuke or two (or two dozens) are exchanged between WARPACT and NATO troops in Germany and Turkey (i'm thinking of the Thrace area). After 2-3 weeks either the Soviets reach the Rhine and ask for an armistice or the war stalemates and ask for an armistice.