Hi r/askhistorians !
I have noticed that in the history of conquests, the status of local nobles/powerbrokers is often discussed. The Norman conquest of England is noted to be one where there was a wholesale replacement of the Anglo-Saxon nobility. Conversely, it seemed that the Visigoth nobles got to remain in power in Iberia even after the Arabic conquests, and Polish nobles were still recognized as nobles after their country's partition by Prussia, Austria, and Russia.
These different approaches fascinated me, and lead to the following questions.
-Why did some conquerors desire to replace the entire aristocracy of a conquered country?
-Conversely, while the maintaining of local order is a common reason for keeping the local nobility in power, what factors caused some conquerors to favor this strategy?
-How common was the elimination or displacement of the local aristocracy? Were there periods in history where this strategy was more common?
-If a province of one empire was conquered by another, but the original empire still existed, what would usually happen? Would they stay in their region, or would they leave? (possible examples could be Swedish nobles in Finland post Russian conquest, Danish nobles in Scania, English nobles in France, ect.)
I'm interested in periods all across history to be honest, apologies if the question is too broad. Any help would be highly appreciated, from any period of time!
Although there's no single unifying answer to the questions above, you might be interested in this older answer of mine where I discuss Spanish rule in Milan, Italy. The answer is part of a broader thread about possessions of the Spanish crown in Europe.
Of course, there are a multitude of other cases and examples that can be added, each with their own unique conditions and restraints. Historic decision makers generally try to make the best decision given the circumstances laid out before them. In Early Modern Lombardy, the aristocracy organized itself early on in the wars of the sixteenth century in order to ensure rule of law as the great powers of Europe contested control over the fragmented Italian peninsula; and the existence of social, economic, and political links between influential political actors in Italy and the monarchies and aristocracies in countries like France and Spain made for an easy linkage between the occupiers and the occupied.
Overall though, I think that the complete overhaul of an entire political-administrative class is very difficult and attempts to do so very often lead to societal collapse and breakdown of law and order; this is true in the modern day, and was most certainly true for pre-contemporary society. While there certainly are examples, I would say they exceptions rather than the rule, and I discussed as much in this older answer. Indeed, territorial conquest much more commonly occurred in bordering territories where social and political systems were already predisposed towards facilitating an easy transition into a new governing order. And even in those situations where territories were not bordering, as say, in the Italian maritime republics or the Aragonese thalassocracy, absorption by a larger polity often occurred when the political climate was favorable.