The Roman Empire at its peak was huge. Despite its size there was relatively easy movement of communication, commerce, and military activities around the empire. How did Rome keep their vast terrories linked together?
The main instrument employed by Rome to keep its empire together and to be able to administer it despite the vast distances involved was its sizable military. The Roman army was a ubiquitous presence throughout the far-flung expanse and military garrisons enforced the dictates of the Roman governors who ruled territories for which they were responsible like fiefdoms. It was that very expanse, and the financial costs associated with administering it, however, that contained the seeds of the empire's eventual destruction. Not for nothing are parallels frequently made between the global diplomatic and military commitments of the United States and the fall of the Roman Empire. Armies are not cheap, especially technically proficient ones. Additionally, and of no small concern, the difficulty of supporting an army spread out over vast distances is considerable. Enemies hoping to weaken Rome's hold on its empire knew to attack its "lines of communication," in effect, the land and maritime routes upon which Rome was dependent for holding those territories. That meant taking measures to secure those routes, which meant additional soldiers, which meant additional financial costs associated with feeding and supplying those soldiers.
One means of helping to maintain a hold on an empire is through the assimilation of conquered peoples into the culture and language of the conquering nation. In other words, make those whose territory has been colonized vested members of the home nation, in this case, Rome. By physically transplanting Romans to distant conquered territories, and by bestowing the benefits of Roman citizenship upon conquered peoples, it is hoped that Roman control over those regions can be consolidated. If colonized peoples can be compelled or enticed to identify with the colonial power, it is presumed, then those peoples will feel a vested interest in maintaining the stability of the territory. This practice of assimilation extends to religious matters, with the religious practices of Rome, including the later adoption of Christianity, imposed upon subject nations.
As noted, the very strength of the Roman Empire was also its most important weakness. By expanding as widely as it did, it made its ability to hold the empire together much more difficult. Military-related expenditures became prohibitively high, and restive captured peoples increasingly rebellious, meaning additional efforts and expenses required to combat insurrection.