Here is a link to 11 events where computer and human error nearly led to nuclear war http://greenerblog.blogspot.com/2015/10/11-instances-where-nuclear-deterrence.html

There is a real chance of nuclear war. All it takes is a combination of some or all of these factors: political tension, military engagement, human error in the command structure and in the group-think in the decision makers, human malice and a computer error  for the fateful decision to be made.

Once one missile is launched, it must be assumed that all available WMDs will be fired, because of the "use them or lose them" imperative. It is naive to hope that reason would prevail once a nuclear exchange is started.

Since the chance of nuclear war is greater than zero, and since the results of nuclear war are unacceptable, it follows that the precursor to nuclear war, i.e. the nuclear deterrence theory, must be abandoned.