Edward Gibbon in The Decline and Fall of the Roman Empire noted:

The narrow policy of preserving, without foreign mixture, the pure blood of the ancient citizens, had checked the fortune and hastened the ruin of Athens and Sparta. The aspiring genius of Rome sacrificed vanity to ambition, and deemed it more prudent, as well as honourable, to adopt virtue and merit for her own wheresoever they were found, among slaves or strangers, enemies or barbarians.

This policy worked, however, only when the encompassed populations were assimilated into the Roman culture. Gibbon continues,

Their partial distinctions were obliterated, and they insensibly coalesced into one great nation, united by language, manners, and civil institutions … So sensible were the Romans of the influence of language over national manners, that it was their most serious care to extend, with the progress of their arms, the use of the Latin tongue … Domestic peace and union were the natural consequences of the moderate and comprehensive policy embraced by the Romans.