94% of China’s population lives to the east of the Hu Line. West of the line is mountainous and less fertile land. To the east is fertile, flat land which gets 15 inches of rainfall per year (the Hu Line follows the 15-inch isohyet).
Read What China wants.
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Protecting the North China Plain is the highest priority in China’s policies both domestic and foreign. The North China Plain is where most of the population lives (the Hu Line) and is the ancestral home for the Han ethnic group. There is also a thousand years of history that includes failing to protect the heartland (e.g. Mongol invasion).