I have a new article in Reason where I review two books on the great empires of history and explain that imperialism does not make the conquering nations any richer or safer. Moreover, decentralized societies are safer than consolidated empires because would-be conquerors cannot take existing administrative systems and technologies and use them for their own ends. Somewhat paradoxically, the stronger we make our own state, the more vulnerable we become to outside conquest because the apparatus can easily be turned against us. If you want to make your society safe from outside invasion, make your own government as weak as possible.
I also interviewed one of the authors, Timothy Parsons, who is a professor at Washington University in Saint Louis and happened to be my history adviser when I studied there as an undergraduate. During our discussion, Parsons describes how the only winners in empires are special interest groups, how empire can corrupt the politics of the metropole (Edmund Burke gets a mention here as an anti-imperialist), and how the conquerors often become the conquered. Parsons argues that empires are no longer feasible because of the rise of national over local identity. Nation-states have largely put an end to empire as it was traditionally known, but nation-states themselves are inherently imperial. According to Parsons, the major difference is that nations seek to turn people into citizens while empires only seek subjects.