The House of Chang during the late Ch'ing Manchu Dynasty--the times of the Opium Wars, the Taiping Rebellion and the ending of that dynasty to the Communist Revolution.
The Ch'ing Manchu Dynasty was also considered to be part of what was called Imperialist China. Imperialist China was reaching out to other countries and states through military force and conquering them so that those countries could be part of the greater "dynasty". Therefore, these countries could now be carved into states and contribute products or wealth to the leaser or emperor of the current dynasty.
The Opium Wars was a time period between 1839 to 1842 as well as 1856 to 1860. The first opium war was based on the monopoly of the trade market by the Chinese through their sale of silk, porcelain and tea. The British as well as the Western colonies felt that their trade was being forced out by the Chinese so they began to sell opium plants to the Chinese in order to recoup their losses. The second opium war began over the British, again, trying to reclaim the opium trade from China.
Since the novel goes up through the 70's, the Communist Revolution in China and Russia colors the rest of the novel's background.