Also, nations that today are considered monoethnic, like France or Spain, have their origins in a more or less violent melting or mixing process.
There is a distinction between a society, a nation, a people, and a state. See multi-national state for the specific political and military issues arising from such a state. There is much overlap however between the concerns of running a state, and finding a common identity as a nation.
Multiethnic societies have existed in various historical contexts such as ancient China, the Roman Empire, or the Austro-Hungarian Empire. In recent times, formation of the United States of America, Yugoslavia, and Czechoslovakia are examples of more or less successful multiethnic societies. Last but not least, establishment of the European Union is such a multiethnic experiment which is still in progress and whose outcome is still uncertain.
The ultimate step in multiethnic integration is the U.N, whose goal is to ensure the peaceful coexistence of as many different countries or ethnic groups as possible.
An apparent important precondition for the success of a multiethnic society is the availability of a common language, as was the case in the Roman Empire, and still is the case in the U.S. Alternatively, several "overlapping" languages, as found in the European Union or Canada, can serve the same function. However, even in the European Union, English is the lingua franca for business and scientific exchange.
An even more important precondition for the functioning of a multiethnic society is an education towards tolerance and understanding. This means not the weak tolerance of those who feel themselves inferior, but the strong tolerance of a self-confident but not proud personality which is able and willing to learn from others without fear of losing its own identity.History of multiethnic societies
Examples of existing multiethnic societies
Examples for destroyed multiethnic societies
Preconditions for the success of multiethnic societies