“Despotism is a legitimate mode of government in dealing with barbarians, provided the end be their improvement, and the means justified by actually effecting that end. Liberty, as a principle, has no application to any state of things anterior to the time when mankind have become capable of being improved by free and equal discussion. Until then, there is nothing for them but implicit obedience to an Akbar or a Charlemagne, if they are so fortunate as to find one. But as soon as mankind have attained the capacity of being guided to their own improvement by conviction or persuasion (a period long since reached in all nations with whom we need here concern ourselves), compulsion, either in the direct form or in that of pains and penalties for non-compliance, is no longer admissible as a means to their own good, and justifiable only for the security of others.” J. S. Mill “On Liberty”

"Liberty as principle has no application to any state of things..." What I feel he's getting at and explaining rather well, is that liberty is the result or outcome not the principle, so the principle would be free and equal discussion, equity and reciprocity. In other words giving the barbarian a seat at the table, this will or should compel improvement or movement towards the outcome of liberty.

I feel what could have been better expressed is that repressive and punitive measure won't get that outcome. Raising the problem, how to give the barbarian a seat at the table in the first place without endangering the achievement of those at the front? A better way to say that might be, how to do it in a manner that doesn't hinder progress or endanger life. Force is the first principle of a barbaric act, so a barbarian is he who uses force to achieve his goals. Force as opposed to persuade or demonstrate. Mill highlights in that, you can't force one to be compelled, or punish non-compliance. The only justification of force is in the "security" or protection of others and oneself.