I'm sure many of you know of Lt. Col.
Stanislav Petrov, the Soviet air defense officer who prevented nuclear war in 1983. He was commanding the Oko early warning station when it falsely "detected" a US missile being launched. Petrov correctly judged it was a false alarm.
Let's say, however, that the duty officer who called in sick and had Petrov take his place hadn't? Suppose Petrov hadn't been there for whatever other reason? Or suppose that he followed protocol and sent the warning?
The Politburo meets in Moscow. With only minutes to decide, they make the decision to retaliate. Nuclear launches are ordered against US and NATO targets.
Then, the report comes in - the system has malfunctioned and it was a false alarm. However, nuclear missiles have already been launched, and are streaking their way. Andropov picks up the hot line and calls Reagan, likely after Reagan has been notified that US early warning stations have picked up the Soviet launch, and explains that yes, Soviet nukes are inbound, but that it was based on a false alarm.
So what do you suppose Reagan's decision, as well as the advice of his cabinet and advisers, would have been? How would it have all ended?