A mindset, in decision science and general systems theory, refers to a set of assumptions, methods and notations that create a powerful incentive to continue to agree with prior conclusions, to use prior tools. It is described as a mental inertia, or "groupthink", or "paradigm" applying to analysis and decision and solutions, and which is hard to escape.
A well-known example is the "Cold War mindset" prevalent in both the US and USSR, which included absolute trust in two-player game theory, in the integrity of command chain, in control of nuclear materials, and in mutual assured destruction of both in the case of a war. This mindset, usefully, served to prevent an attack by either.
Most theorists consider challenging the assumptions of an embedded power group to be the key responsibility of the power group itself. Power groups that fail to challenge their own assumptions regularly enough, cannot hold power, as no mindset is flexible enough to apply for all future events. The variations in mindset between Democratic and Republican Presidents, for instance, may have made the USA more able to challenge assumptions than the Kremlin with its more static bureaucracy.
Modern military theory challenges that mindset and requires dealing with asymmetric warfare, terrorism and proliferating weapons of mass destruction. In combination, these represent "a revolution in military affairs" and require very rapid adaptation to new threats and circumstances. The huge costs of implementing another mindset may be small compared to that of discovering it.