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It comes from a poem of Tu Fu who was a very famous poet of the Tang Dynasty.

In 756 AD, the rebel commander Yin Ziqi led an army to lay siege against the strategic city of Suiyang. The defending commander, Zhang Xun, believed that if he could take out the leader the rebel's morale would sink and he would be able to launch a counter attack. He devised a plan with his best archers. The next time the rebels assailed the wall they were to shoot back using the branches of trees. When Yin Ziqi heard that the defenders were reduced to shooting with branches he felt certain the city was ready to be taken. Before the next assault he moved in closer to better oversee the final victory. Riding atop his horse he unknowingly came within range of the archers who had saved their arrows for just such a moment. One arrow hit Yin Ziqi in the left eye killing him instantly. The spectacle of their commander's death in front of almost the entire rebel army served to demoralize them to such an extent that they dispersed the field.