Story on Duke Xiang of Song (宋襄公): In the Battle of Hongshui (泓水) against the much stronger and fully prepared enemy, Xiang’s troops were defeated thoroughly and he himself was badly hurt and died in the following year (637 BC). Earlier, his brother had advised him to attack when the soldiers of the Chu State were crossing the river. Xiang announced his benevolence and righteousness, insisting on waiting until the enemy completely crossed the river. “We are not Duke Xiang of Song,” Chinese leader Mao Zedong commented in his On Protracted War (No.83), “and have no use of his asinine ethics” (我们不是宋襄公，不要那种蠢猪式的仁义道德). Which principle of Sun Tzu for river warfare did Duke Xiang violate leading to the catastrophe for his kingdom?