Morgenthau became professor at the University of Chicago. Along with E.H. Carr, he is one of the main authors of the realist school in the 20th century. This school of thought holds that nation-states are the main actors in international relations, and that the main concern of the field is the study of power.
His book Politics Among Nations defined the field of international relations theory in 1948 as it heralded the post–World War II paradigm shift in American thinking about diplomacy. Politics Among Nations emphasized the power interests of states as the driver behind the relations between states. The period before WWII was on the other hand defined by idealism that focused on values.
While 'realism' is used generically to refer to a closely associated, and growing, body of work; it has taken various forms since Thucydides' 'History of the Pelopponesian War'. The realism developed by Morgenthau and his contemporary E H. Carr is referred to however as Classical Realism, not to be mistaken of course for it's close relative Neo-Realism.
The theoretical aims of Classical Realism: to introduce a conceptual framework that helps us understand:
1) What happened in the first half of the 20th century? => a crisis of the western civilization
2) Why did it happen? => because of a general decay in western political thinking (the dominance of liberalism)
3) What can be done to prevent it from happening again? => The introduction of a better theory and practice of politics (political realism).
Note: the main reason why political realism is related to a Thucydidean philosophy is its contention that a deficiency of a philosophical nature leads to political crisis.
The main dictum of 'liberalism' as described in Morgenthau's "scientific man vs. power politics" is the belief in the redeeming powers of science and reason. In this depiction, liberalism is a school of thought that believes in the transformation of man through education in order to solve social problems. It was articulated through the historical rise of the middle classes. Liberalism has misunderstood the nature of man since it believed that society, pretty much like nature, is subject to specific general laws. What it did not contemplate was the intrinsic human nature that rendered human affairs completely unforeseeable. Morgenthau contends that in politics, there are no fixed formulae, it is not a technical question as liberals take it to be. It is the wisdom of the statesman who operates in this environment of uncertainty that counts. The moral responsibility of the politician appears exactly at this point where the statesman uses the 'art of statecraft', knowing that there is no pre-established certainty of success.
1. Political realism believes that politics is governed by objective laws with roots in human nature.
2. The main signpost of political realism is the concept of interest defined in terms of power which infuses rational order into the subject matter of politics, and thus makes the theoretical understanding of politics possible.
3. Realism assumes that interest defined as power is an objective category which is universally valid but not with a meaning that is fixed once and for all. Power is the control of man over man.
4. Political realism is aware of the moral significance of political action. It is also aware of the tension between the moral command and the requirements of successful political action.
5. Political realism refuses to identify the moral aspirations of a particular nation with the moral laws that govern the universe. It is the concept of interest defined in terms of power that saves us from moral excess and political folly.
6. The political realist maintains the autonomy of the political sphere; he asks "How does this policy affect the power of the nation?" Political realism is based on a pluralistic conception of human nature. A man who was nothing but "political man" would be a beast, for he would be completely lacking in moral restraints. But, in order to develop an autonomous theory of political behaviour, "political man" must be abstracted from other aspects of human nature.