“Never let any Government imagine that it can choose perfectly safe courses… Prudence consists in knowing how to distinguish the character of troubles, and for choice to take the lesser evil.”

Source: The Prince (1513), Ch. 3 (as translated by RM Adams). Variants [these can seem to generalize the circumstances in ways that the translation above does not.]: The Romans, foreseeing troubles, dealt with them at once, and, even to avoid a war, would not let them come to a head, for they knew that war is not to be avoided, but is only put off to the advantage of others.
There is no avoiding war; it can only be postponed to the advantage of others.
Context: The Romans never allowed a trouble spot to remain simply to avoid going to war over it, because they knew that wars don't just go away, they are only postponed to someone else's advantage. Therefore, they made war with Philip and Antiochus in Greece, in order not to have to fight them in Italy... They never went by that saying which you constantly hear from the wiseacres of our day, that time heals all things. They trusted rather their own character and prudence — knowing perfectly well that time contains the seeds of all things, good as well as bad.

As quoted in Margaret Mead : Some Personal Views (1979) edited by Rhoda Métraux
As quoted in American Quotations (1992) by Gorton Carruth and Eugene H. Ehrlich
1970s
Variant: At times it may be necessary temporarily to accept a lesser evil, but one must never label a necessary evil as good.

Source: The Last Wish

Source: The Virtue of Selfishness: A New Concept of Egoism

In Cold Blood (1965)

Source: Danse Macabre

Fragments of Markham's notes
The Nemesis of Faith (1849)
Context: It is alike self-contradictory and contrary to experience, that a man of two goods should choose the lesser, knowing it at the time to be the lesser. Observe, I say, at the time of action. We are complex, and therefore, in our natural state, inconsistent, beings, and the opinion of this hour need not be the opinion of the next. It may be different before the temptation appear; it may return to be different after the temptation is passed; the nearness or distance of objects may alter their relative magnitude, or appetite or passion may obscure the reflecting power, and give a temporary impulsive force to a particular side of our nature. But, uniformly, given a particular condition of a man's nature, and given a number of possible courses, his action is as necessarily determined into the course best corresponding to that condition, as a bar of steel suspended between two magnets is determined towards the most powerful. It may go reluctantly, for it will still feel the attraction of the weaker magnet, but it will still obey the strongest, and must obey. What we call knowing a man's character, is knowing how he will act in such and such conditions. The better we know him the more surely we can prophesy. If we know him perfectly, we are certain.