The world economy in the ninth century had two powerful engines. One was Tang dynasty China, an empire stretching from the South China Sea to the borders of Persia, with ports open to foreign traders from far and wide. The Tang welcomed diverse people to its capital, Changan, the site of modern-day Xian, and multiethnic groups lived side by side in a city of a million—a population unmatched by a Western city until London in the early 19th century. Then, as today, China was an economic powerhouse—and much of that power was built on trade.
The other economic engine was Baghdad, capital of the Abbasid dynasty from 762 onward. That dynasty inherited the Muslim world in the Middle East; by 750 it had spread as far as the Indus River to the east and Spain to the west, bringing with it trade, commerce, and the religion of Islam (the Prophet Muhammad himself had been a merchant).