Schools in realism
There are different doctrines or schools of realism but classical realism and neorealism or structural realism can be defined, sometimes even neoclassical faction is mentioned.
Some 30 years ago, classical realism was mostly replaced by neorealism. Historical thinkers of classical realism were, inter alia, Thucydides, Machiavelli and Hobbes. They were followed by Hans Morgenthau, Thomas Schelling and others in the 20th century. Neorealism/structural realism is represented by authors like Kenneth Waltz and John Mearsheimer.
Structural realism explains power and security competition between great powers mainly by the factors of international system. Anarchy, polarity (number of great powers in the system), amount of might among great powers, distribution of power as well as the changes in the power structure are central factors in the international politics. These structural factors standardize and conform to the behavior of great powers.
All great powers act according to the same logic regardless of their political system or different administration. According to the balance of power theory of Waltz, the anarchic character of international system will re-balance the imbalance of the system and bipolar system is more stable than multipolar system. Waltz’s balance of power theory and structural realism enabled the scientific explanation of international politics and power competition of great powers. International environment defines the interests and behavior of great powers in the structural realism. Realism has attempted to explain the patterns in international politics by looking at the structure of the system rather than at its units, the states.
Structural realism has had so far quite little to say about a unipolar world, other than that it will soon come to an end. Waltz (1979) sets the smallest number of great powers at two, thus excluding unipolarity from the scope of his analysis. In a 2011 interview he acknowledged this omission but has described unipolarity as “altered bipolarity” or an anomaly that would soon disappear. Enduring unipolarity challenges one of the basic predictions of structural realism as it stands today, the balance of power.
In early 90’s, Jack Snyder proposed and used first time the division of realism into aggressive and defensive factions. This division is based on the fundamental conceptual differences between these factions. According to defensive realism the anarchy of international system is not necessary bringing conflicts and wars between states. The total picture of international scene is more optimistic and tendencies towards balance of powers are encouraging great powers on moderate competition. Some prominent writers in this group are Stephen Walt, Charles Glaser, Stephen van Evera, Robert Jervis and Shiping Tang.
According to aggressive realism, the international system promotes persistent power and security competition between great powers as well as aggressive behavior against each other. Maximizing of military power is the best way to maintain the security of the state. The main study of this faction is The Tragedy of Great Power Politics by John Mearsheimer.
According to the neoclassical faction, the international system has indirect effects on the power politics of great powers. The foreign policy of great powers is decided in internal /domestic operational environment. Therefore, the factors at the state level and inside the state shall be studied and evaluated taking into account the international structural factors. Some prominent scholars in this group are Gideon Rose, Randall Schweller, William Wohlforth and Jeffrey Taliaferro.