If you go to ancient Rome to look for signs of modern nationalism, I'm sorry but I see little point of continuing this discussion any further.
That is my point. This isn't modern nationalism. It's nationalism, something that has been around forever and will be around until we invent hive minds. Some countries are unified ethnically and religiously, and some countries aren't. Those that aren't are going to have a tough time holding together, requiring extraordinarily charismatic leaders and general prosperity. War is certainly a major factor in the survival of a multiethnic empire, but it is far from the end-all, be-all you claim it is.