A study by Campbell (2007 p. 28) shows that International Relations date back to Greek history based on sovereign states. Prior to this, most of the nations used hierarchical religion orders, especially from the Roman Empires. However, by 1789, most nations had acquired independence and became sovereign states or nation-states as opposed to former religious and monarchy states. This European system extended to America, Asia, and Africa through colonization in the name of setting standards of civilization. During the Cold War, decolonization finally established the contemporary international system. However, some post-modern nations have disputed levels of analysis on individual levels, international levels, domestic units, and global levels. International relations came after the First World War with IR theories drawing on the work of other social sciences.
International relations theories divide into two categories namely the positivists and post-positivist epistemological camps. Positive theories involve the social sciences through analyzing the material forces like the size of military forces, state interactions, and power balances among others. On the other hand, post-positivists reject the fact that world study happens through objective and value free manner (Halliday 2005 p. 66). This epistemology does not believe in the national-choice theory arguing that the social world cannot apply social science in its study. The main difference between these two epistemologies is that while positivists have casual explanations on why and how different nations exercise power. The post-positivists argue from constitutive questions like the meaning of power, its components, and reproduction among others. Their approach applies ethics, unlike the positivists.