The idea of deterrence is that the promise of mutual destruction is sufficient to prevent nuclear war. What Scott Sagan argues is that nuclear-armed states are unfortunately not rational single players in a game (in game-theoretic terms). He argues that the control of nuclear weapons is in the arms of large hierarchical organizations (i.e. governments). As such, their behavior is unstable and “noisy”. For example, a false alarm may lead to escalation with the spread of misinformation or misinterpreted information up the chain of command.
What is the scenario that he is so concerned with? Something like the Cold War. Where two or more states walk up to the edge of the cliff in order to threaten that they may jump. And since states are not individual people but chaotic organizations, they are much more likely to accidentally slip once they are standing at the edge of that cliff.
All this, of course, is ignoring the elephant in the room which is a terrorist organization acquiring a nuclear weapon (through theft or some back-door deal). From the cold hard perspective of probability, it seems inevitable that this will happen sometime in the future. Given that people do not learn from history, I can’t help but think that we will be reminded of the destruction that a nuclear weapon can bring once we forget, and we will forget, we always do.