Wathching the fire on the other side of the river.

It's used to describe: Delay entering the field of battle until all the other players have become exhausted fighting amongst themselves. Then go in full strength and pick up the pieces.

Towards the end of the Eastern Han dynasty, Cao Cao defeated Yuan Shao's forces in the Battle of Guandu. After Yuan Shao's death, his three sons fought among themselves in a battle of succession.

Cao Cao took advantage of the chaos and attacked, but the brothers united against their common enemy.

Cao Cao's strategist, Guo Jia suggested, "The Yuan brothers were fighting among themselves and our attack only caused them to unite against us. If we retreat, they'll fight among themselves again. Once they are weakened, it'll be easy to win."

After Cao Cao's troops withdrew, the Yuan brothers fought among themselves for control of Fen province. The eldest brother Yuan Tan was angry because their father had made the second son Yuan Shang his heir. The youngest son Yuan Xi supported Yuan Shang.

In the meantime, Cao Cao conquered Qing, Fen, Bing and You provinces. Yuan Tan was killed. Yuan Shang and Yuan Xi fled to Liaodong in the north-east to seek the help of nomadic chieftain Gongsun Kang. Instead of pursuing the Yuan brothers, Cao Cao summoned his forces back saying that Gongsun Kang would send the heads of the Yuan brothers to him.

After some time, Gongsun Kang indeed sent the heads of the two Yuan brothers to Cao Cao. Cao Cao later explained, "Gongsun Kang was afraid he'd be my next target; he was also afraid that the Yuan brothers would annex his territory. If I attacked him, he would need the help of the brothers. But if I withdrew... Tough luck for the brothers.