But when night came, the water in the Yong River suddenly rose; yet the Chu army still tried to stealthily cross the river according to the marks made in the daytime. As a result, over 1,000 soldiers were drowned. There was utter chaos of desperate cries and shoutings on the river. All at once, the army collapsed like a fallen building, and was routed.

Once upon a time, the State of Chu planned to attack the State of Song. But the Yong River was the natural barrier against its surprise attack at night.

According to the marks, the Chu army would have been able to cross the Yong River during the day; but the rising water changed the situation in the night. Yet the Chu army still tried to wade across the river according to the marks made in the daytime, so naturally they failed.

To overcome this obstacle, the State of Chu sent men to secretly measure the depth of the Yong River and make marks for wading across the river at night.