General Discussion

We assume that statesmen think and act in terms of interest defined as
power, and the evidence of history bears that assumption out. That assumption
allows us to retrace and anticipate, as it were, the steps a statesman-
past, present, or future-has taken or will take on the political scene.
We look over his shoulder when he writes his dispatches; we listen in on
his conversation with other statesmen; we read and anticipate his very
thoughts. Thinking in terms of interest defined as power, we think as he
does, and as disinterested observers we understand his thoughts and actions
perhaps better than he, the actor on the political scene, does himself.

The concept of interest defined as power imposes intellectual discipline
upon the observer, infuses rational order into the subject matter of politics,
and thus makes the theoretical understanding of politics possible. On the
side of the actor, it provides for rational discipline in action and creates that
astounding continuity in foreign policy which makes American, British, or
Russian foreign policy appear as an intelligible, rational continuum, by and
large consistent within itself, regardless of the different motives, preferences,
and intellectual and moral qualities of successive statesmen. A realist theory
of international politics, then, will guard against two popular fallacies:
the concern with motives and the concern with ideological preferences.

~snip~

Yet even if we had access to the real motives of statesmen, that knowledge
would help us little in understanding foreign policies, and might well
lead us astray. It is true that the knowledge of the statesman's motives may
give us one among many clues as to what the direction of his foreign policy
might be. It cannot give us, however, the one clue by which to predict his
foreign policies. History shows no exact and necessary correlation between
the quallty of motives and the quality of foreign policy. This is true in both
moral and political terms.

We cannot conclude from the good intentions of a statesman that his
foreign policies will be either morally praiseworthy or politically successful.
Judging his motives, we can say that he will not intentionally pursue
policies that are morally wrong, but we can say nothing about the probability
of their success. If we want to know the moral and political qualities
of his actions, we must know them, not his motives. How often have
statesmen been motivated by the desire to improve the world, and ended
by making it worse? And how often have they sought one goal, and ended
by achieving something they neither expected nor desired?

Morgenthau, H. (1948). Politics among nations: The struggle for power and peace (pp. 5, 6). New York: Knopf