Does he offer any clarification for how he defines wealth, significant or occuppied? The statement is pretty broad and could mean quite a few things. My initial thoughts would be that the Mongol conquest of the Jin or the conquest of the Ming by the Jurchen would qualify. But, it is hard to say without knowing what he meant exactly.

By the end of WW2 I'd question the assertion that the Germans were wealthier than the Russians. Their country was in tattered ruins.

Like many of the others, I feel like the author is a bit vague about things. First off, though I don't doubt that the average soviet citizen was poorer than the average German citizen, the countries as a whole had a very similar GDP at the beginning of the war, though Germany did have more "industrial" resources such as iron and coal. By the end of the war, I don't think anyone could seriously claim that Germany was richer than the Soviet Union, as they (the soviets) were the second most powerful nation on the planet.

Basically any instance you can think of when "barbarians" invaded a more established land.

Wealth is of course something somewhat vague in the eras before our interconnected economic system of today. But I would classify "cultivated fertile land" and infrastructure as wealth, especially in the days when people carried very little if any currency for the most part.

The Mongols certainly were a steppe people with generally few possessions of the sort that we would normally consider to be 'wealth', and they overran ancient Chinese cities that had been centers of civilization for centuries.

Some might argue that the Spanish conquistadors overran a rather wealthy Tenochitlan. Certainly it was a bigger and more opulent city than Spain could really boast of.