"There seems to be no little need that the whole doctrine of non-interference with foreign nations should be reconsidered, if it can be said to have as yet been considered as a really moral question at all... To go to war for an idea, if the war is aggressive, not defensive, is as criminal as to go to war for territory or revenue; for it is as little justifiable to force our ideas on other people, as to compel them to submit to our will in any other respect. But there assuredly are cases in which it is allowable to go to war, without having been ourselves attacked, or threatened with attack; and it is very important that nations should make up their minds in time, as to what these cases are... 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..."
According to Mill's opinion (in 1859) barbarous peoples were found in Algeria and India where the French and British armies had been involved. Mill's justification of intervention was overt imperialism. First, he argued that with "barbarians" there is no hope for "reciprocity", an international fundamental. Second, barbarians are apt to benefit from civilized intervenors, said Mill, citing Roman conquests of Gaul, Spain, Numidia and Dacia. 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."
While seeming wildly out of kilter with modern discourse, a similar approach can be found in theory on intervention in failed states. Of more widespread relevance, Mill discussed the position between "civilized peoples".
"The disputed question is that of interfering in the regulation of another country’s internal concerns; the question whether a nation is justified in taking part, on either side, in the civil wars or party contests of another: and chiefly, whether it may justifiably aid the people of another country in struggling for liberty; or may impose on a country any particular government or institutions, either as being best for the country itself, or as necessary for the security of its neighbours.
Mill brushes over the situation of intervening on the side of governments who are trying to oppress an uprising of their own, saying "government which needs foreign support to enforce obedience from its own citizens, is one which ought not to exist". In the case however of a civil war, where both parties seem at fault, Mill argues that third parties are entitled to demand that the conflicts shall cease. He then moves to the more contentious situation of wars for liberation.
"When the contest is only with native rulers, and with such native strength as those rulers can enlist in their defence, the answer I should give to the question of the legitimacy of intervention is, as a general rule, No. The reason is, that there can seldom be anything approaching to assurance that intervention, even if successful, would be for the good of the people themselves. The only test possessing any real value, of a people’s having become fit for popular institutions, is that they, or a sufficient portion of them to prevail in the contest, are willing to brave labour and danger for their liberation. I know all that may be said, I know it may be urged that the virtues of freemen cannot be learnt in the school of slavery, and that if a people are not fit for freedom, to have any chance of becoming so they must first be free. And this would be conclusive, if the intervention recommended would really give them freedom. But the evil is, that if they have not sufficient love of liberty to be able to wrest it from merely domestic oppressors, the liberty which is bestowed on them by other hands than their own, will have nothing real, nothing permanent. No people ever was and remained free, but because it was determined to be so..."
Libertarian historian Joseph R. Stromberg states that J.S. Mill's imperialistic views are incompatible with his alleged liberalism, since maintenance of the British Empire would require government repression .