Before 600 c. e., large centralized empires, such as the han, persian and roman empires the extended their military power by:
a. giving more political power to the common people in conquered territories, thus eliminating the need for large armies
b. developing supply lines and building infrastructure, including defensive walls and roads.
c. creating open societies inclusive of different religions and cultural practices, thus decreasing the chance of revolts
d. recruiting their armies entirely from inhabitants of the pie core teritorries and excluding members of newly conquered lands