Reddit mentions of Imagined Communities: Reflections on the Origin and Spread of Nationalism

I agree that a universal human characteristic is to operate in groups, and therefore to regulate group membership. In general, before the modern period, group identity was smaller and more locally based. Religion was one way for people to think of themselves as belonging to the same imagined community as distant, unknowable people, but local identities and loyalties would have been more important.

One of the really important developments in making possible nations is printing and the rise of newspapers and other print media. For one thing, these standardize language to a certain extent, allowing people who once would have had mutually unintelligible dialects to communicate with a common grapholect. More generally, it puts into contact distant people, and makes it possible to imagine a "we" that encompasses people and places that you will never know, never see. Once you can begin to image that kind of "we," it is possible to begin constructing a history of that we, an origin story, a set of characteristics which that we is said to share, and a "they" who are not "we"--perhaps even precisely the opposite of "we." That, then, is how modern nations get made, in a nutshell.

Check out Benedict Anderson's Imagined Communities. It's a really influential book, because one of the most important trends in historiography in the last few decades is the recognition that nations are not timeless, ahistorical, or natural, and the subsequent historical investigation of the creation of idea of the nation. (And amazon.com claims you can get a copy for a penny. Totally worth it.)

Edit: I'm sure an ancient historian or two will point out that Roman identity was quite extensive geographically, and one could likely make some interesting comparisons between Roman identity and modern national identity. The account I've given is largely one that takes places in modern Europe. It will be interesting to see how scholars of the ancient world or other parts of the world critique this--what we might call--model of nationalism.