First, technology and production. The critical technological factor in the intensifying warfare of the period from about 500 to 200 BC was iron. Bronze is made of mainly of copper, which is relatively easy to extract and work. Iron is harder to get at than copper, and melts at a higher temperature, so it postdates bronze in human technological history. But it is much more common in the earth, so once the technology was developed, it meant that more people could have metal tools and weapons. Ironworking first came to Central Asia in about 1000 BC (and reached Africa about the same time).5 Small iron objects have been excavated from today’s Xinjiang in the far west of China from about 900 BC, and iron-smelting had come to Zhou lands by about 800 BC. After about 400 BC, iron replaced bronze in weaponry, and was used for tools. Iron was cheaper and tougher than bronze. It meant higher productivity of land and labor: that is, fewer people could grow more grain on the same amount of land, cut down trees more quickly, and so on. Iron tools meant that the population of commoners grew, and more soldiers could be fed; iron weapons meant that more soldiers could be armed. Both developments contributed to the increasing warfare of the time.
Qin was, in archaeologist Arthur Cotterell’s words, “exceptionally apt” at picking up and promulgating the production and use of iron, as well as other technology.6 In 350, Qin moved its capital eastwards to Xianyang, right in the center of the Guanzhong plain, and encouraged people to clear new farmland and create irrigation canals.7 Key to Qin’s conquest were the work of laborers and the brains of an engineer, Li Bing. As a Qin official in Sichuan, from about 250- 240 BC, Li Bing persuaded Sichuan natives and Qin colonists to construct a remarkable irrigation system in which a levee divided the Min river into two halves. One of the two streams acted as a flood channel and carried boat traffic; the other branched into numerous small streams and canals that carried water into a network of thousands of tiny irrigation ditches. Li harnessed a natural force to push water through the irrigation system: he knew that where a river bends, the water on the outside of the curve will travel faster than on the inside. The system, now called Dujiangyan, turned the Sichuan basin around Chengdu into a bread basket that fueled the Qin conquest and is still working today.
Dujiangyan requires annual upkeep by the families and communities of the area, but compared with its benefits the work is minimal, and it has been done century after century. In the Han period (in 2 CE), because the system made the area so wealthy, more people lived in Chengdu than anywhere else except the imperial capital, Chang’an. One source listed Chengdu’s products in the Han period:
The earth grows the five grains; the sacrificial victims include all six domestic animals. Mulberry trees, silkworms, hemp, ramie, fish, salt, bronze, iron, cinnabar, lacquer, tea, and honey; the divine turtle, the great rhinoceros, long tailed pheasant, silver pheasant; shimmering golden fabrics and bright cosmetics – all these they offer as tribute.”8
A twelfth-century scholar marveled at the prosperity Li Bing’s system still brought the Chengdu plain:
There are grand houses in the City of Pi; every house has running water and tall bamboo… There are thousands of big bamboos, dripping thick and green… Early in the morning I made a brief stop at Ande Town… In one direction the river divides and flows into all the [irrigation] ditches with a thunderous sound, curling up like snow, as the beautiful fields fill the eyes.9
Because of Sichuan’s wealth, it often became a kingdom unto itself as imperial dynasties faltered. The system still feeds people now. It rebounded from destruction in the 1640s and the Great Leap Forward in the 1950s, but present-day urbanization and larger farms are challenging its health. The Dujiangyan system gave Qin, a northern state, access to southern rice with which to feed its many soldiers, armed with iron.