Feudalism was simply a trade of securities: the lords provide protection to the serfs, while the serfs pledge their lives to their lords. Finis. Imperial systems weren’t much different: any large-scale “trade” had to exist within the borders of the empire. The only way to secure access to new goods was to venture out and conquer. And since any advantage would be temporary, it all came down to the security-for-loyalty trade of the Imperial Center to its provinces, as guaranteed by imperial armies.
