The Han Dynasty, with a brief interregnum, lasted for over 400 years, and its longevity derived from a solid system of administrative control. The standard unit of local government was the commandery, each headed by an administrator reporting in to a provincial governor or inspector. Interwoven with the commanderies were a network of kingdoms and marquessates, ruled by loyal relatives of the emperor, thus thwarting the build-up of regional power bases by any potential rival. State monopolies of salt, iron and manufacture of coinage financed military expenditure and ambitious building programmes. In 111 BCE, the empire was extended to the Gulf of Tonkin in the south, while on the northern frontiers a century of warfare saw the warlike, nomadic Xiongnu reduced to vassals (51 BCE), and the borderlands seeded with agricultural colonies. The protectorate of Sinkiang, established in 119 BCE, effectively opened the northern Silk Road.
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