I'm from New York City, and as such, I regularly encounter and deal with people of many nationalities, religions, and cultures. Some of them are involved in local, state, and federal government entities, and in our day-to-day lives, we get along very well.
From time to time, it occurs to me how shocking and weird this ought to be. Most countries, throughout history, have had one dominant race and culture (unless I am mistaken--in that case, please do correct me). What's weirdest of all is that now the commander-in-chief of our military is a mixed race guy, and the military itself is a very diverse organization, and is at the same time one of the most respected organizations in the US.
Yet we are not entirely historically unprecedented. The Mughals, the Romans, and many other massive Empires managed to have multinational societies and militaries, and for a time, each of them managed to survive and leave behind proud legacies. So my question to you, /r/AskHistorians, is how they managed to do it. How can you run a country full of people with very little in common? How do you motivate soldiers of different races and religious backgrounds to fight together under one flag?
I welcome all answers and would also appreciate it if you pointed me to books that address this issue. Many thanks in advance.
This question is incredibly broad, but I'll try to give a basic understand of state building and in doing so use the Ottoman Empire as an example. Firstly you have to understand which time period each empire exists in. For the Ottoman Empire, from around 1261-1920. Then you need to examine the movements, evolution of thought, and the concept of citizenry. For the Ottoman Empire, a state spanning ~650 years, there were a multitude of movements both for and against the state powers.
While each empire or state uses different methods of controlling its foreign populace, all these processes are designed to mollify, reduce the power of, or to fidelify (fake word don't use this) the foreign populations within its territory. Take the Ottoman Empire, they utilized the Janissary corps, devsirme, and the Timar system to do all three of the concepts I mentioned previously. By taking young Christian boys from families in the outlying regions and Islamicizing them the Ottomans brought these minority groups into the fold. This leads into the concept of the Janissary corps by taking these newly Muslim converts and training them to be the elite Sultanic guard, tying them directly to the leadership of the state. The Timar system often held a stable life for many Janissaries, calvarymen, and even sometimes Ottoman slaves, giving people the idea that the Ottoman empire cared for the minorities.
In an empire it is important to avoid the repression of local cultures, and the Ottoman Empire tended to do just that. Although not exactly a historical academic, Mark Mazower writes a fascinating timeline of Salonica, today known as Thessaloniki, and the approach utilized by the Islamic Ottoman Empire. Instead of removing all traces of Christianity and Judaism, the Ottomans actually allowed many of the churches and synagogues to continue functioning in relative freedom from the Muslim leadership. Although conquered, the Ottoman Empire managed to avoid direct confrontation with the populace by allowing their local culture to survive and even thrive in some cities. Even in the great city of Constantinople, now known as Istanbul, many of the churches were left unconverted by Mehmed. He even went so far as to replicate the architecture of the converted Hagia Sofia in the Great Imperial Mosques that would be constructed over the next two centuries.
Along with the relative unmolested lifestyle of the religious and ethnic minorities, the Ottoman Empire took it a step further by restarting the Millet system, something that had briefly existed under the Sassanid Empire about 2,000 years previously. The Millet system used a multitude of courts that handled "personal law" cases in which two members of the same religious community were in conflict. Although any time a muslim was involved it went to the courts that dealt in Sharia law, it illustrated a cohesive understanding of statebuilding. Furthermore, since these millets were separated by religion and not ethnicity, it helped bond different ethnic minority groups together.
This is just one empire during one time period. There is not definite one right answer for this question, but rather you should look at a multitude of empires and try to find the commonalities amongst them. I believe that absolving ethnic boundaries, respecting local cultures, and allowing minorities to prosper while creating a cohesive national identity are paramount to the civility and stability of any great multi-national empire.
Sources:
Mazower, Mark Salonica: City of Ghosts
Goffman, Daniel The Ottoman Empire and Early Modern Europe
Not sure about the others but the Austo-Hungarian empire would pit its smaller constituents against one another. Meaning it would seek to use the rivalry's that already existed culturally in the regions and inflame them so that the people would fight each other than revolt against the empire itself.