With stable empires in Europe and China as well as countless small republics in various alliances in Central and Southern Asia, trade between East and West is intense and increases permanently throughout the 1st milennium CE. It requires solution for safety on the Silk Road. The solution found is the greatest international co-operation in history so far.
Stable, non-expansive empires and a different security infrastructure for global trade were just two among many factors benefitting city states as predominant political model in the second half of the 1st milennium CE - instead of territorial feudalist kingdoms.
Rome`s South-Eastern neighbour, the Kingdom of Aksum, adopted Christianity as a state religion, remained a monarchy and became an expanding empire in the 4th-8th centuries before its internal political structures changed and assimilated to a greater extent to the successful Roman model.