"Conventional empires, such as the first Roman Empire, the British Empire, or the current American Empire, typically come into existence when the imperial centre develops such a surplus of demographic and/or economic and/or technological resources that it is in a position to bring large tracts of territory into line either by conquest ('formal empire') or by intimidatory regimes of stick and carrot ('informal empire'). These kinds of empire will generally last only as long as the advantage in resources is maintained, plus maybe a couple extra generations thanks to force of habit, before its disappearance is recognised, and they are overturned. It was precisely because a new equality in levels of development across the European landscape had made the old Roman-type empire impossible by the end of the first millennium, that the new Papal Roman Empire came into existence, and the kinds of advantage that create more normal empires usually are time-limited, not least because the act of imperial projection tends anyway to erode them. This was certainly the case with Europe in the first millennium, and is arguably the case now, where America and the West have encouraged massive economic expansion in Asia for their own purposes, but created in the process what is likely to become the next world superpower."

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"Limited as it certainly was in political and military terms, the papacy certainly created an empire nonetheless, and in some important ways a much more powerful and oppressive one than the first Romans had ever managed. The projection of their imperial values never got past the landowning elites, where their papal successors targeted the entirety of the population" - Peter Heather, The Restoration of Rome