What's the core idea of historical materialism which the Communist Manifesto starts out with? All history hitherto is the history of class struggle. We're only developing new scales for old methods of exploitation. Romans lacked heavy industry but they had factories with a division of labour reflecting them. They lacked the modern economic relationships that created the bourgeoisie but their patricians served the same role in society. Their colonialism was driven by primitive accumulation, a core population backed by a powerful military and industrial/logistical network which demands ever-greater levels of comfort. They needed Egyptian grain to distribute Caesar's bread, they needed British tin for bronze in regions without tin deposits.

Capitalism evolved out of the new division of labour that developed when aristocracy gave way to a merchant and bureaucrat class. A lord and serf had the same boss-employee relationship in a form reflecting the village and castle, but merchants could afford larger factories backed by larger resource extractors and the way power was wielded shifted away from the old institutions. The forms of modern colonialism increasingly became shaped by the bourgeoisie that profited from it instead of the older aristocratic forms. It's just a more complex manifestation of what has been happening since the Sumerians.