Zhang Renyuan

Zhang Renyuan built three fortified cities beyond the Yellow River, the Cities of Surrender. He closed the road south.^[1]

The raids

Kapgan Khan raided the Tang frontier for twenty years. The raids worked because the river could be crossed where nobody was waiting. The plunder could be taken back across before a Chinese army came up. Armies sent after raiders arrive late by design.

The three cities

Three garrisons on the far bank changed what a raid cost. A raiding force now had to pass fortified points on the way in and on the way out. Its captured stock slowed it down on the return.

It is the only measure on the Chinese side of the wars with Kapgan that can be shown to have worked. It is a piece of engineering rather than a victory. The steppe had no answer to walls it could not besiege and did not want to hold.