China and India together account for one-third of humanity. Both were advanced civilizations when Europe was in the Dark Ages. Until the 19th century, they constituted the world’s largest economies. Today they are, in terms of purchasing power, the world’s largest and third-largest national economies, and the fastest-growing major economies. Were they to form an alliance, they would dominate mainland Eurasia and the sea lanes of the Indian Ocean and Western Pacific that carry a preponderance of the world’s maritime energy trade. Yet these civilization-states seem destined to compete in the 21st century.