Battle of Changshao

The Battle of Changshao was a military conflict between Qi and Lu, two major principality states in Shandong Peninsula during the Spring and Autumn Period of Zhou dynasty. The battle happened in the January (lunar calendar) of 684 BC at Changshao. Lu claimed the victory under the lead of general Cao Gui (曹刿). This battle is described in detail by Zuo Qiuming in his Spring and Autumn Commentary of Zuo, but it is not mentioned in Sima Qian's book of Shi Ji. This battle is considered as a classic example of winning by good military strategies, and the widely-used Chengyu of Yi Gu Zuo Qi was derived from this battle.

Background
Duke Xiang of Qi was assassinated in 686 BC; one month later, his successor was also assassinated. Duke Xiang's younger son, Xiaobai, took the throne in the spring of 685 BC as the Duke Huan of Qi. Duke Huan's elder brother, Lord Jiu, believed that he was the right person to take the throne, so he allied with Duke Zhuang of Lu and launched a war against Duke Huan. However, the army of State of Lu was defeated by Qi in the Battle of Ganshi, and Duke Zhuang of Lu was almost killed in the battle. Realizing that State of Lu was supporting Lord Jiu, Duke Huan of Qi was annoyed. He planned to attack Lu as a revenge.

The Battle
In the spring of 684 BC, Duke Huan of Qi ordered his army to attack Lu. Cao Gui was appointed as the general of Lu to defend the state. Duke Zhuang of Lu also presented at the frontline along with Cao Gui. Cao Gui asked the army of Lu to stand still in the battle formation and fortifications and not to fight back until his order. The army of Qi charged twice but the army of Lu stayed in the fortifications and did not respond. When the Qi soldiers started their third charge, Cao Gui ordered the army of Lu to launch a counter attack. At this point, soldiers of Qi was tired and could hardly concentrate, so they were defeated. Duke Zhuang of Lu immediately ordered soldiers of Lu to chase the fleeing Qi army. Cao Gui accepted the order after making sure that there was no ambush. Qi army was completely destroyed and expelled out of the border.