Is Cosmopolitanism Inherently Unstable?

One of the founding fathers of sociology, Ferdinand Tonnies discussed at length the inherent tension between communities (groups based on a shared identity) and societies (groups based on diversity). This came to mind this morning listening to Philip Mansel discuss his new book Splendour And Catastrophy On The Mediterranean on the BBC. He examines the rise and fall of Smyrna and Alexandria as a context in which to consider contemporary Beirut, a focus of cosmopolitanism in the contemporary Middle East.

It led me to wonder whether cosmopolitanism is an unstable social state. The joining of cultures with the possibilities for growth and synthesis is a major catalyst in social evolution. The times and places that have been the most cosmopolitan have also, by in large, been the most fruitful for human society. We do better when we do what we do with each other.

But it has to happen somewhere and therefore therefore cosmopolitanism always happens on someone's home turf. When things turn bad politically, socially, or economically, which they are bound to do eventually, it tends to give rise to a reflexive isolationism and scapegoating. If things are worse than usual, what is the cause? What is different? Oh, those people over there. They must be to blame. Of course, they are no doubt having a tough time of it also and then when the suspicion, the alienation, and especially the legal or economic measure are put into place to punish them, they become bitter and you get a circling of the wagons counter-movement that stresses purity within their community which may launch anything from separatist movements to terrorist attacks like the one we saw in Sweden.

Is this sort of dynamic unavoidable? How does one protect cosmopolitanism from the inherent dangers?