There are two other reasons why it's harder to conquer countries now than then.

1. people are more squeamish now; using Nazi levels of brutality in putting down rebellions would attract consideral moral opprobrium

2. wealth now comes from highly skilled information workers, who must be at least partly free if they are to be productive; whereas in the past wealth came from farmland, raw materials, and unskilled labourers. The level of repression necessary to hold a territory (and people) against its will is likely to make the acquired terroritory unproductive.