For at least the past thirty years, scholarship on international relations has been bewitched by a simple proposition: the polarity of the international system is a central cause of great power strategies and politics. The number of poles (dominant countries) in the system is like an invisible fence that shapes states as if they were dogs with electronic collars or a Skinner box that conditions national rats. States can choose to ignore the fence or box, but if they do, they must pay the consequences. The polarity of the international system as defined by the number of great powers - involving more than two (multipolarity), two (bipolarity), or one (unipolarity) - is expected to mold states and international politics in different predictable ways. The central place of polarity in IR theory is such that it is commonly assumed that the appropriate way to study the world is to examine the impact of polarity first and then move on to other lesser factors to mop up any unexplained variance
