Islam started as a religion that grew into an empire while the Tang Dynasty started as a political institution and adopted religious principles and administrative ideas. It was established by the so called prophet Muhammed, while the Tang Dynasty was established in 618 A.D. by Li Yuan, after the fall of the Han. The Tang had a Cosmopolitan culture, which absorbed many new cultural elements and allowed different religions to thrive without persecution generally, although Buddhism was persecuted at times. In Islam it was different; for religious freedom one must pay a price. The religious minority in the Islamic world, the Jews and Christians, were accepted as long as they accepted Islam’s ultimate political rule and paid special taxes. When they did this they were not so persecuted for their beliefs and left mostly to deal with their own disputes and religious affairs. So while both the Tang and Islam were religiously pluralistic, the Islamic world made it harder for the minority religions and religious factions to survive. This fact helped to unify the Tang Dynasty and tear apart the Islamic world.
The Tang introduced many new tactics into their reign, while also drawing on the Han practices. They expanded bureaucracy and tightened imperial control over governors. They established a professionally trained aristocratic cavalry. They introduced imperial nomination. And, although they did accommodate new religious institutions, they called upon Confucian teaching to unify their political culture. In the Islamic World they did not draw on any old religious or political ties, they created a new government.
There were many political and religious splits within Islam: first the Sunnis and the Shiites, then a split between East and West and also the split between the educated and the commoners. The Sunnis thought that the line of succession was through the four rightly guided Caliphs and then through the Umayad and Abbasid Dynasties. The Shiites on the other hand thought that only one of the four rightly guided caliphs was the correct air; Muhammed’s son in law, Ali. They believed that his descendants were the rightful airs to power. The Shiite way appealed to the groups that were left out of the Umayadd and Abbasid Dynasties. In the east the Abbasid empire was in the hands of a series of Caliphs with no real political or religious influences. Non-arabs were able to lead during this time. The West though, was ruled by the Arabs, although non-Arab Berbers represented an important minority. Sufi brotherhoods helped convert urban populations and peasants, and Heterogeniety within Islam reflected the ability of the Sufi brotherhoods to reach out to the common people, but the Ulama spoke to the educated and scholarly through sharia. From 1050 to 1300 no single political or ethnic group was strong enough to hold sway over the Islamic core area.
At it’s peak the Tang ruled a larger empire than the Han or the Islamic world even though it lasted only 300 years. The growth of Islam did not bring local populations together under a common ruler, rather these efforts to centralize led to a series of defeats by outsiders. Thus although both empires rose and fell at times, the Tang was much more successful in creating a single unified empire.