Razib writes,

I think it is critical to emphasize why ancient barbarian elites were so keen on conquering civilized states, and why there seems to have been less mass migration of the peasantry. In the modern world when we think of differences between societies in regards to wealth, complexity or glory, we consider the median man on the street. This would tell us little for most of human history, rather, we would have to focus on the top 10% to truly get a sense of the difference, and in particular the top 1%. To a great extent civilization has been a racket which operates to the benefit of the tiny elite by making rent-seeking much more efficient.

That is how I am inclined to look at it. Where archeologists see trade, I see theft. But that, as you know, is my most wrong view.