Why did Chang’an require grain and supplies from eastern China and the Yangtze River valley?

Chang’an, the ancient capital of the Han, Sui, and Tang dynasties, required grain and supplies from eastern China and the Yangtze River valley primarily to sustain its large urban population and support its standing armies.12

While North China served as the political center for successive dynasties, the Yangtze River basin was of vital economic importance due to its immense agricultural potential. Because the capital cities in the north often lacked sufficient local production to feed their concentrated populations and military forces, imperial regimes relied on transporting surplus grain from the agriculturally productive southern river valleys. This necessity drove the development of major infrastructure, such as the Grand Canal, which was specifically built to facilitate the movement of these essential resources from the south to the northern capitals.32