no international authority and states are left to their own devices to ensure their own security. Realists believe that sovereign states are the principal actors in the international system. International institutions, non-governmental organizations, multinational corporations, individuals and other sub-state or trans-state actors are viewed as having little independent influence. States are inherently aggressive (offensive realism) and obsessed with security (defensive realism). Territorial expansion is only constrained by opposing powers. This aggressive build-up, however, leads to a security dilemma whereby increasing one's security may bring along even greater instability as an opposing power builds up its own arms in response (an arms race). Thus, security becomes a zero-sum game where only relative gains can be made. Realists believe that there are no universal principles with which all states may guide their actions. Instead, a state must always be aware of the actions of the states around it and must use a pragmatic approach to resolve problems as they arise. A lack of certainty regarding intentions prompts mistrust and competition between states. Rather than assume that states are the central actors, some realists, such as William Wohlforth and Randall Schweller refer instead to "groups" as the key actors of interest. Finally, states are sometimes described as "billiard balls" or "black boxes". This analogy is meant to underscore the secondary importance of internal state dynamics and decisionmaking in realist models, in stark contrast to bureaucratic or individual-level theories of international relations. Realism in statecraft The ideas behind George F. Kennan's work as a diplomat and diplomatic historian remain relevant to the debate over American foreign policy, which since the 19th century has been characterized by a shift from the Founding Fathers' realist school to the idealistic or Wilsonian school of international relations. In the realist tradition, security is based on the principle of a balance of power and the reliance on morality as the sole determining factor in statecraft is considered impractical. According to the Wilsonian approach, on the other hand, the spread of democracy abroad as a foreign policy is key and morals are universally valid. During the Presidency of Bill Clinton, American diplomacy reflected the Wilsonian school to such a degree that those in favor of the realist approach likened Clinton's policies to social work. Some argue that in Kennan's view of American diplomacy, based on the realist approach, such apparent moralism without regard to the realities of power and the