Disunity is caused by greed for power. Since the beginning of our time each clan have been greedy for power. It's a common problem among all ethnic groups. (See Japan, Korea, China, etc... for more examples.)Without a strong national identity or a super culture to absorb the rest then you cannot overcome such. Hence for China to have created their identity and unity Mao did away with all religon, and nearly all cultures. Once the national identity, China was established and its people was reformed, little by little more freedom was given because without freedom, you will not have a culture, and without a culture, once the central government falls, another super culture will absorb you. Possibly Japan, Europe or USA. With a strong national identity and culture, it is incredibly hard to be absorb. If you look back to Roman time, even though they conquor Greek, their culture was so influence by Greek that it appeared they continue the Greek culture to a new level, and to now, it seems like the USA is merely a continuation of modern Rome.