Realism (international relations)
Realism in international relations is a doctrine that assumes that it may be entirely possible to work with otherwise hostile states when matters of common concern are involved, but one of the constraints is that the internal politics of a nation is not a matter of common concern and out of scope for the proposed cooperation. While Henry Kissinger is its most visible advocate, he was by no means the originator. The driver of states, in realist theory, is external power.
A cynic might liken it to the international version of the Serenity Prayer: "give me the serenity to accept the things I cannot change, the courage to change the things I can change, and the wisdom to know the difference."
Hans Morgenthau, indeed, starts simply: "We assume that statesmen think and act in terms of interest defined as power...A realist theory of international politics, then, will guard against two popular fallacies: the concern with motives and the concern with ideological preferences" 
Kissinger had addressed the basic ideas going back to his senior thesis, which eventually became the book, A World Restored, which explored "balance of power" explanations for stability in early 19th century Europe.  In the Nixon Administration, his first priority was replacing the containment policy against the Soviet Union and Chinea with détente, or a "lessening of tensions". Detente tends to specific to key issues, where realism has broader scope. It was a change from the containment policy, which expected to restrain and indeed weaken opposing power blocs through economic and psychological warfare. Detente, or the "lessening of tensions", assumed that the opponent could not be significantly weakened, but also accepted they would cooperate on mmaggers of common concern. It is a subset of realism (foreign policy), which further recognizes that states can have reason to cooperate on external policies, but it often would be impractical to change their internal practices.
More recently, he refined the definition over Morgenthau: "In fact, the United States is probably the only country in which the term "realist" can be used as a pejorative epithet. No serious realist should claim that power is its own justification. No idealist should imply that power is irrelevant to the spread of ideals...Realists judge policy by the ability to persevere in the pursuit of an objective in stages, each of which is imperfect by absolute standards but would not be attempted in the absence of absolute values.
"The acolytes of idealism sweep away such restraints; focusing on the ultimate objective, they reject the contingent discussion of feasibility with its inevitable geopolitical component.
"Realists seek equilibrium; idealists strive for conversion. This is why crusaders have usually caused more upheavals and suffering than statesmen."
"States are the primary, indeed only the only important actors in world affairs, the relation among states is one of anarchy, and hence to insure their survival and security, states invariably attempt to maximize their power...it [may attempt] to protect its own security by strengthening its power and/or allying with other states." Huntington sees this as one of several systems without end, as opposed to that of his pupil, Francis Fukuyama, who speaks of an "end to history".
- International security, in an insecure world, comes from maintaining a balance of power with one's potential enemies; this requires the ability to defend oneself by military means, without help from others
- Friendship and enmity with states depends ontheir power, not their ideology or their internal governance
- Capabilities, rather than intentions, is the basis for assessing threat
- Moralism has no place in foreign policy
He distinguishes between two kinds of realism:, 
- a characteristic of the force-dominated relationship beween "historical" and "post-historical" actors, which he considers accurate
- an assumption that all states seek to maintain insecurity and to seek power, which he does not consider a natural, objective drive.
He believes that realism may yield to the widespread evolution of liberal democracy.
- Hans J. Morgenthau (1978), Six Principles of Political Realism, Politics Among Nations: The Struggle for Power and Peace (Fifth Edition, Revised ed.), Alfred A. Knopf, pp. 4-15
- Henry Kissinger (1973), A World Restored: Metternich, Castlereagh and the Problems of Peace, 1812-1822, Mariner Books, ISBN 0395172292
- Henry Kissinger (May 12, 2005), "Realists vs. idealists", New York Times
- Samuel P. Huntington (1996). The Clash of Civilizations and the Remaking of World Order. Simon & Schuster. ,p. 33
- Francis Fukuyama (1992), The End of History and the Last Man, Free Press, ISBN 0029109752, pp. 248-250}}
- Fukuyama, pp. 278-279