The myth of a monolithic China was shattered this past week. Running barely beneath the surface of what the government has sought to portray as a “harmonious” society, the fracture created by the Urumqi and Lhasa riots threatens to shake the country.

Foreigners and the Chinese themselves typically picture China’s population as a vast Han majority with a sprinkling of exotic minorities living along the country’s borders. This understates China’s tremendous cultural, geographic, and linguistic diversity—in particular the important...