Slaves. This is why so many patricians in Rome funded and raised on their own money legions. So they could invade distant lands and return slaves to rome.
And the other important aspect was the trading. It kept the empire homogenuous and wealthy, in all its parts.
The Roman economy relied heavily on slavery, trade, military conquest, taxation, and infrastructure development. Slavery provided the labor force, while trade networks connected regions for economic growth. Military conquests expanded resources and taxation supported public works and military funding, ensuring stability in the empire.
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