Thanks for the quote and source. I believe Corpus Juris Civilis was based on existing law so the concept goes back much further, and I would guess was incorporated into Islamic empire's came from Roman.

> The difference between European empires and Islamic/Roman ones would be what JumpCrissCross advanced + the extent to which the conquered inhabitants are incorporated into the state, no?

Is it not rather more complex than that? The Roman Empire eventually granted citizenship to conquered people's but after centuries and gradually - all free men getting citizenship was 3rd century. When initially conquered a lot of people were incorporated into the state as slaves. AFAIK the Islamic empires were similar, and the price of being treated equally was to adopt the conquerors culture and religion.

The European Empire I am most familiar with (the British) only wanted the ruling class of its colonies to adopt its culture (with consequences such as speaking fluent/native English being a class marker that last to this day). It also (at least later on) gave colonies increasing autonomy.