“Political realism refuses to identify the moral aspirations of a particular nation with the moral laws that govern the universe.”

The Spirit of Christianity and its Fate (1799)

Six Principles of Political Realism, § 5.
Politics Among Nations (1948)
Context: Political realism refuses to identify the moral aspirations of a particular nation with the moral laws that govern the universe. As it distinguishes between truth and opinion, so it distinguishes between truth and idolatry. All nations are tempted — and few have been able to resist the power for long — to clothe their own aspirations and action in the moral purposes of the universe. To know that nations are subject to the moral law is one thing, while to pretend to know with certainty what is good and evil in the relations among nations is quite another. There is a world of difference between the belief that all nations stand under the judgment of God, inscrutable to the human mind, and the blasphemous conviction that God is always on one's side and that what one wills oneself cannot fail to be willed by God also.

Six Principles of Political Realism http://www.mtholyoke.edu/acad/intrel/morg6.htm, § 1.
Politics Among Nations (1948)
Context: Political realism believes that politics, like society in general, is governed by objective laws that have their roots in human nature. In order to improve society it is first necessary to understand the laws by which society lives. The operation of these laws being impervious to our preferences, men will challenge them only at the risk of failure.
Realism, believing as it does in the objectivity of the laws of politics, must also believe in the possibility of developing a rational theory that reflects, however imperfectly and one-sidedly, these objective laws. It believes also, then, in the possibility of distinguishing in politics between truth and opinion — between what is true objectively and rationally, supported by evidence and illuminated by reason, and what is only a subjective judgment, divorced from the facts as they are and informed by prejudice and wishful thinking.

Ten Sermons of Religion (1853), III : Of Justice and the Conscience https://en.wikisource.org/wiki/Ten_Sermons_of_Religion/Of_Justice_and_the_Conscience
Context: Justice is the constitution or fundamental law of the moral universe, the law of right, a rule of conduct for man in all his moral relations. Accordingly all human affairs must be subject to that as the law paramount; what is right agrees therewith and stands, what is wrong conflicts and falls. Private cohesions of self-love, of friendship, or of patriotism, must all be subordinate to this universal gravitation towards the eternal right.

Six Principles of Political Realism, § 4.
Politics Among Nations (1948)
Context: Realism maintains that universal moral principles cannot be applied to the actions of states in their abstract universal formulation, but that they must be filtered through the concrete circumstances of time and place. The individual may say for himself: "Fiat justitia, pereat mundus (Let justice be done, even if the world perish)," but the state has no right to say so in the name of those who are in its care. Both individual and state must judge political action by universal moral principles, such as that of liberty. Yet while the individual has a moral right to sacrifice himself in defense of such a moral principle, the state has no right to let its moral disapprobation of the infringement of liberty get in the way of successful political action, itself inspired by the moral principle of national survival.

1950s, Rediscovering Lost Values (1954)

Source: The Soul of Liberty (1980), p. 103

In Quest of Democracy (1991)