The presence of different nations under the same sovereignty is similar in its effect to the independence of the Church in the State. It provides against the servility which flourishes under the shadow of a single authority, by balancing interests, multiplying associations, and giving to the subject the restraint and support of a combined opinion. In the same way it promotes independence by forming definite groups of public opinion, and by affording a great source and centre of political sentiments, and of notions of duty not derived from the sovereign will. Liberty provokes diversity, and diversity preserves liberty by supplying the means of organisation. All those portions of law which govern the relations of men with each other, and regulate social life, are the varying result of national custom and the creation of private society.  In these things, therefore, the several nations will differ from each other; for they themselves have produced them, and they do not owe them to the State which rules them all. This diversity in the same State is a firm barrier against the intrusion of the government beyond the political sphere which is common to all into the social department which escapes legislation and is ruled by spontaneous laws. This sort of interference is characteristic of an absolute government, and is sure to provoke a reaction, and finally a remedy. That intolerance of social freedom which is natural to absolutism is sure to find a corrective in the national diversities, which no other force could so efficiently provide. The co-existence of several nations under the same State is a test, as well as the best security of its freedom. It is also one of the chief instruments of civilisation; and, as such, it is in the natural and providential order, and indicates a state of greater advancement than the national unity which is the ideal of modern liberalism. Lord Acton Nationality
Lord Acton (1834-1902), who is primarily known for his pithy quotes, will never run the risk of becoming a hero of progressive thought. But alerted by Jacob Levy, I decided to take a look at him. This post is a kind of homage to Levy, whose already classic essay on the constructive role of so-called 'identity politics' inspires and anticipates substantially my reflections here. In the very next paragraph from the one just quoted he explains how "inferior races" will benefit from being surrounded by and mixing with the superior kind. (This recalls Berkeley's racial eugenics for Ireland.) Although interestingly enough, it seems his racial theory seems to rely on a natural life-cycle of racial vitality such that racial inferiority and superiority are not essential and stable characteristics. And, while he seems to have thought that slavery was immoral (see, for example, here and his remarkable encomium to the Essenes), even his editors agree that his support for the South in the American civil war was not free from ambiguity on the point of slavery. In addition, in the very essay that is under discussion slavery is treated as a live option functionally without a hint of moral disapproval (again, not to say it is endorsed). In addition, Acton has a fondness for empire--the whole essay is a critique of the despotic risks immanent in the nation-state (as advocated, say, by Mazzini). And the passage quoted is really a defense of multi-national empire (of the sort familiar from Austria, Russia, and, of course, Acton's own Britain) in the name of a providential conception of history.
In what follows, I will treat Acton's remarks about nations to think about what we would call cultures. The way I re-interpret Acton here is that while we tend to think naturally (and so encouraged by modern mechanics and economics) that efficiency is superior to friction, there are, in fact, important foreseeable byproducts of social friction that are worth cherishing even encouraging. The second and third sentences of the quoted passage ("It provides against the servility which...of notions of duty not derived from the sovereign will") echo Madisonian ideas in favor of a large republic (Federalist 10), although the emphasis on preventing servility strikes me as relatively original to Acton. But the key idea here is that Liberty provokes diversity, and diversity preserves liberty by supplying the means of organisation.
The way I understand Action there are two kinds of friction in a multi-cultural environment: first, friction among groups, and second, friction between the state and some (minority)groups. The first friction is the product of different ways of living and multiple opinions in society. The second friction is also the product of different ways of living and multiple opinions in society, but is deepened because one group (or the state) wishes to impose its views on others and this generates foreseeable further friction. The second friction creates, once laws are imposed, political mobilization ("organisation'') and a counter-reaction to the state's authority.
It's this diffuse capacity to mobilization that is, in fact, a key argument for a liberal society more generally. That is, shared culture (or nation for Acton) is a form of (let's use economicsy jargon) social capital (due to willingness of shared sacrifice, mutual trust, etc.) that can be deployed to try to check oppression. He is, undoubtedly, too confident that a 'remedy' (the language betrays commitment to theodicy) will be found.
In a way, Acton helps us supplement the standard libertarian argument (found in, say, Milton Friedman) about the benefits of market economies. This argument, in brief, goes that even if market economies concentrate wealth among some sub-set of the population (that's an open question revived by Piketty), dispersed wealth still provides powerful sources of resistance to government power. What Acton recognizes is that there are non-monetary sources that also provide powerful means to mobilize resistance (obviously financial and cultural sources of capital can strengthen or interact with each other). The history of civil and identity rights movements suggests that such friction and mobilization is indeed a non-trivial element in the progress of liberty.