Book excerpt: "Empires are not an under-researched topic. Recently, there has been a veritable surge in comparative and conceptual studies, not least of pre-modern empires. The distant past can tell us much about the fates of empires that may still be relevant today, and contemporary historians as well as the general public are generally aware of that. Tracing the general development of an empire, we can discern a kind imperial dynamic which follows the momentum of expansion, relies on the structures and achievements of the formative period for a while, and tends to be caught in a downward spiral at some point. Yet single cases differ so much that a general model is hardly ever sufficient.There is in fact little consensus about what exactly constitutes an empire, and it has become standard in publications about empires to note the profusion of definitions.Some refer to size-for instance, 'greater than a million square kilometers', as Peter Turchin suggested. Apart from that, many scholars offer more or less extensive lists of qualitative criteria. Some of these criteria reflect the imperial dynamic, for instance, the imposition of some kind of unity through 'an imperial project', which allows moving broad populations 'from coercion through co-optation to cooperation and identification'"--