Dated 1874, a year after his death, but it does sound exactly like something he would say:
To suppose that the same international customs, and the same rules of international morality, can obtain between one civilized nation and another, and between civilized nations and barbarians, is a grave error, and one which no statesman can fall into, however it may be with those, who from a safe and unresponsable position, criticize statesmen.
Among the many reasons why the same rules cannot be applicable to situations so different, the two following are among the most important. In the first place, the rules of ordinary international morality imply reciprocity. But barbarians will not reciprocate. They cannot be depended on for observing any rules. Their minds are not capable of so great an effort, nor their will sufficiently under the influence of distant motives. In the next place, nations which are still barbarous have not got beyond the period during which it is likely to be for their benefit that they should be conquered and held in subjection by foreigners. Independence and nationality, so essential to the due growth and development of a people further advanced in improvement, are generally impediments to theirs. The sacred duties which civilized nations owe to the independence and nationality of each other are not binding towards those to whom nationality and independence are either a certain evil, or, at best, a questionable good. The Romans were not the most clean-handed of conquerors; yet would it have been better for Gaul and Spain, Numidia and Dacia, never to have formed part of the Roman Empire?
To characterize any conduct whatever towards a barbarous people as a violation of the law of nations, only shows that he who so speaks has never considered the subject. A violation of great principles of morality it may easily be, but barbarians have no rights as a nation, except a right to such treatment as may, at the earliest possible period, fit them for becoming one. The only moral laws for the relation between a civilized and a barbarous government are the universal rules of morality between man and man.