Religion is one. The situation in Northern Ireland is a good example. The Protestants and Catholics have been at each other's throats for centuries. Another case is the Hindus and Muslims of India. There are now sporadic clashes between the two nations and each has nuclear weapons pointing at the other.
Language is also divisive.
Our neighbor to the north serves as an example. French-speaking Quebec wants to succeed from Canada, and I've been told that speaking English in Quebec can get a traveler in trouble. But the difference need not be so much as an entire language. The best way to botch an interview for a professional position is to use the politically incorrect word or incorrect grammar. But if any nation on earth can overcome the inertia of suspicion and bigotry, it's the United States.