Hans Morgenthau, Politics Among Nations, 3^rd. ed., 1960, p.34

Hans Morgenthau and Kenneth Thompson, Politics Among Nations, 6^th. ed., 1985, p.39

(The text is identical, emphasis added)

Regardless of particular social conditions, the decisive argument against the opinion that the struggle for power on the international scene is a mere historical accident must be derived from the nature of domestic politics. The essence of international politics is identical with its domestic counterpart. Both domestic and international politics are a struggle for power, modified only by the different conditions under which this struggle takes place in the domestic and international spheres.

The tendency to dominate, in particular, is an element of all human associations, from the family through fraternal and professional associations and local political organizations, to the state. On the family level, the typical conflict between the mother-in-law and her child’s spouse is in its essence a struggle for power, the defense of an established power position against the attempt to establish a new one. As such it foreshadows the conflict on the international scene between the policies of the status quo and the policies of imperialism….