Nothing, actually.
But with the events in 1985 and 1986 (Chernenkos death, ASAT, Geneva Summit, Challenger, Mir, Chernobyl...) this makes (with the appliance of some artistic freedom at least) for a quite decent backstory - something which most other games that portait a fictive World War III suck at.
Granted, there would have been many other opportunities for war breaking out in Europe during the Cold War, but during most of the time NATO ground forces would have been a mere nuisance to the Soviet - not something to worry about. This started to change only in the late 70s/early 80s, so the 80s are a natural choice for an interactive version. It wouldn't be fun to try fighting the Soviets with odds so heavily biased as up until 1980.