Six hundred miles west of Hang-chou the Yangtze River emerges from a mountain corridor onto the lowland plain of south-central China. Here the river bed widens sharply and loses depth; the Yangtze winds a snake-like path across a thousand miles of rich, flat, water-laced country, northeastward to Han-yang, then south toward the Po-yang Lake, north again to Chiang-ning (modern Nanking), and finally into the East China Sea at modern Shanghai. Medieval Chinese called it simply “River”—a River among rivers, the designation of an entire S-shaped area whose character depended more on water than on land, the dominant feature of the southern landscape. In the twelfth century this river region developed a strategic importance as it became first the refuge for a besieged court, then an area of contention with barbarian invaders, and finally the principal artery of Chinese control. This transformation reflected a major reorientation of the Chinese mind.