Policymakers and scholars have to deal with the difficult problems of variation, change, and transitions in world politics. Practitioners have to estimate the capabilities and intentions of those with whom they are interacting and need to determine the kind and extent of variety in their environments. Detecting, diagnosing, and dealing with change also is particular difficult. Scholars have shown the wide range of units and systems that human beings have created, but need to also examine the extent to which they are characterized by common processes and dynamics. The balance of power generally operates through unintended consequences and can characterize systems even when no one seeks balance. Change may be more common than scholars often appreciate. Nuclear weapons undermine much traditional international politics and even greater changes will flow from the fact that the leading powers in the world no longer contemplate war with each other.