The early Cold War took place during a time in which atomic weapons had to be loaded onto bombers, and which only had yields in the dozens of kiloton range. It was only after the introduction of hydrogen bombs and ICBMs that people began to consider the end of the world as a plausible outcome for a war between NATO and the Warsaw Pact.

By the time "blow up the world" could have been shorthand for a full nuclear exchange with a doubtful outcome for human civilization, both NATO and the USSR were in a very different place, in terms of military technology, than they had been at the end of the WW2. That being the case, with little evidence of how both sides would fare with their updated equipment and tactics, how did NATO planners go about assessing the probable outcomes of a major conventional clash in Europe? Prior to WW2, many generals profoundly misunderstood the tank, it's implications, limitations, and how to best use it. With the introduction of new weapons like attack helicopters, ATGMs, and so on, how could war planners not avoid the same mistakes that were made in assessing how a war with second generation tanks would be fought?

To boil it down: how did they decide that the conventional conflict would last for less than a week before NATO found it necessary to deploy tactical nuclear weapons (with a plausible escalation to a strategic exchange occurring almost immediately thereafter)?