Classical realism in international relations stems from the international politics of ancient Greece while neorealism has proven to be the dominant theory of international relations since the end of the Second World War.

Classical realism is based on the progressive, liberal thought of idealism whereas neorealism emerged following the brute force political currents that were influential in the rise of national socialism in Germany in the 1930's and the consolidation of authoritarian political currents.

Classical realism emphasizes human nature and the self-interest of states, while neorealism emphasizes that the traits of states and statesmen will determine the balance of power in international relations.

Neorealism abandons the human nature aspect and statecraft ethics that are dominant in the classical realism approach to international relations, instead determining that the international systemic structure as a whole is determinant as it may constrain individual decision makers.

Classical realists see international relations as a result of external pressures shaping state policy and international relations, while neorealism implies that internal forces result in power struggles that shape international relations.