War is naturally characterized by uncertainty, and humans are known to exhibit an in-built optimism bias that frequently causes them to overestimate the likelihood of positive outcomes. This bias may have evolutionarily adaptive advantages in many situations. Yet in the dialectic between military planners generating coercive options within available means and national cabinets seeking solutions to intractable diplomatic or geostrategic problems within acceptable costs, optimism bias can lead to tragic and avoidable outcomes.