How did Cyrus the Great and Darius I each rule diverse groups of people peacefully?

The ethno cultural diversity of the various nations beneath the decree of Persia, its massive geographic extent, and the relentless scuffle for authority by regional contenders, the establishment of a skilled army was compulsory for the preservation of the peace, and also to impose the power of the king in circumstances of revolution and foreign menace.

Explanation:

according to classical republican political theory, which democratic republics would function best?

a.   small republics, with people who had similar interests

b.   large republics, with people who had competing interests

c.   republics made up entirely of landowners

d.   republics made up entirely of servants