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| China population density Source: Forbes |
Everything east of this line has more
than 15 inches of rainfall per year. Everything to the west does not. To the
east of the line developed stable, agricultural society. To the west developed
people who got their food from nomadic practices.
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| China population Source: Forbes |
The 15-inch isohyet, as it is called,
is a major factor in Chinese history. Chinese military garrisons that advanced
west of the isohyet had to be supplied by food from east of the isohyet as the
territory west of the isohyet was of little value. This was a drain on the
Chinese economy. The most important question of Chinese history from the
Eastern Zhou to the Song is whether it was cheaper to bribe the nomads to not
raid China or to lead military campaigns against them.