After ascending the throne, Duke Huan of Qi decisively defeated the state of Lu, which had supported his rival Prince Jiu in a provocative campaign, and forced Lu to execute Prince Jiu, thereby solidifying his rule. He appointed Guan Zhong as his prime minister, and with Guan Zhong's guidance, Qi made significant progress in politics, economy, and military affairs. Seeing Qi's social stability, material prosperity, and strong army, Duke Huan decided to launch a campaign against Lu to avenge the old grievance over Prince Jiu's claim to the throne from a year earlier, while also seeking to conquer Lu and expand Qi's influence outward. However, Guan Zhong believed the time for hegemony had not yet come and advised Duke Huan to focus on internal governance and build alliances with other states, continuing to wait for the right moment. But Duke Huan, eager to achieve dominance, completely ignored Guan Zhong's advice.
In the spring of 684 BC, Duke Huan of Qi sent troops to attack the State of Lu, and the Qi army invaded Lu's territory.
Both the State of Qi and the State of Lu were important vassal states enfeoffed during the Western Zhou Dynasty. The capital of Lu was Qufu (present-day Qufu, Shandong), and its territory mainly covered the southwestern part of present-day Shandong. It was relatively traditional, strictly adhering to the ritual and music system of the Zhou royal society, and by the Spring and Autumn period, it was only considered a second-tier vassal state. In contrast, Qi was the fief of Jiang Ziya, a founding hero of the Western Zhou, with its capital at Linzi (present-day northeast of Zibo, Shandong), governing a vast area in the northern part of present-day Shandong. The land there was fertile and rich in resources. Jiang Ziya and his successors worked diligently to govern, implementing correct development policies, which led to a prosperous economy and strong national power. From the Western Zhou through the Spring and Autumn period, Qi always held a pivotal position among the vassal states. Compared to Qi, Lu was at a disadvantage in both territory and national strength.
However, one year earlier, after the State of Lu suffered a major defeat by the State of Qi, Duke Zhuang of Lu, in order to guard against a Qi offensive, strengthened military training, rushed the production of various weapons, and simultaneously reformed domestic governance to win the people's trust. When Duke Huan of Qi led his army to approach Lu's borders, Duke Zhuang decided to mobilize the nation's forces to resist the Qi attack. The Qi army was strong and well-equipped, and having defeated Lu the previous year, their morale was high. To temporarily avoid the Qi army's momentum, Duke Zhuang led his forces to retreat to Changshao (present-day northern outskirts of Qufu, Shandong), a location favorable for a counterattack.
Just as Duke Zhuang of Lu was preparing to engage in battle, a commoner named Cao Gui from the state of Lu requested an audience with the duke to offer strategic advice. Cao Gui was highly resourceful and possessed genuine political foresight, and driven by a determination to save his country, he went to meet Duke Zhuang of Lu.
Cao Gui asked Duke Zhuang, "What do you rely on to fight against the state of Qi?" The Duke replied, "Things like clothing and food are essential for life; I dare not keep them for myself alone but always share them with others." Cao Gui said, "These are merely small favors, and they cannot reach all the people of the state; the people will not fight with all their might." The Duke said, "In sacrifices, I am always sincere to the gods and dare not falsely report the number of offerings." Cao Gui said, "This sincerity is hard to convince others and may not move the gods; the gods will not bless you because of it." The Duke said, "For all civil and criminal cases, large and small, though I cannot examine every detail, I always handle them with reason and fairness, ensuring no one suffers injustice." Then Cao Gui said, "This indeed fulfills the duty of a ruler; the people can follow you, and you can go to battle." Following this, Cao Gui volunteered and requested to accompany the Duke to the battlefield.
Thus, Duke Zhuang and Cao Gui rode together in the same chariot to Changshao, where the two armies faced off. During the Zhou Dynasty, battles between armies followed strict military etiquette: both sides would line up their formations, advance at the sound of drums, halt after fifty paces to reorganize their ranks, and only when the two forces clashed and charged into the enemy's lines would the formations begin to break apart.
After both armies had formed their battle lines, Duke Zhuang of Lu saw the Qi army attacking the Lu positions and immediately wanted to order a counterattack. Cao Gui advised against it, saying: "The Qi army's morale is at its peak; we are not certain of victory and cannot strike." Thus, Duke Zhuang ordered the Lu army not to attack, only allowing archers to defend and stabilize the formation. The Qi army was full of fighting spirit but had no opponent to engage, and was suppressed by the arrows, unable to break into the Lu positions, so they had to retreat. After a brief rest, the Qi army launched a second attack. This time, Cao Gui again insisted that Duke Zhuang not attack, but hold the formation and defend. The Qi army charged fiercely, but with no one to meet them and still blocked by arrows, they could not enter the formation and had to fall back again. The Qi generals thought the Lu army was too scared to fight. So, they quickly launched a third, more imposing attack. After the first two assaults, the Qi soldiers were exhausted and their morale had waned. However, the Lu army remained orderly and high-spirited. Seeing the battlefield situation had changed, Cao Gui believed the time to strike had come and immediately advised Duke Zhuang to order a counterattack. Duke Zhuang personally beat the war drums and commanded the attack. The Lu soldiers, already eager for battle, charged forward with unstoppable vigor upon hearing the order. The Qi army was thoroughly defeated, retreating step by step.
On the battlefield, the Lu army achieved a decisive victory. Duke Zhuang ordered a pursuit to capitalize on their success. However, Cao Gui advised the duke not to rush the chase. He then climbed onto a chariot to survey the direction of the Qi army's retreat, got down to inspect the battlefield, and only then allowed Duke Zhuang to issue the order to pursue. With high morale, the Lu army relentlessly chased the Qi forces, driving them out of the state's borders and capturing a large number of armored soldiers and supplies, thus securing victory in the Battle of Changshao.
After the victory of the Lu army, Duke Zhuang of Lu could no longer restrain himself and discussed with Cao Gui the reasons for winning the battle. Cao Gui said: "In warfare, one must rely on courage. When the drums are beaten for the first time to order an advance, the courage is at its peak; the second time, the courage has already weakened; and by the third time, the courage is exhausted. The enemy had beaten their drums three times, their morale was depleted, while our army had just beaten the drums for the first time, our morale was at its height. 'Their strength was exhausted, ours was abundant'—how could we not win?" As for why they did not immediately pursue, Cao Gui explained: "Qi is a large state, its military strength has always been formidable. I feared they might be feigning retreat to set an ambush. When I climbed the chariot and saw their banners and weapons scattered in disarray, and when I dismounted and saw the chaotic tracks left by their war chariots, I then concluded that they had truly been defeated, and thus agreed to order the pursuit." After hearing this, Duke Zhuang of Lu was sincerely convinced and nodded in agreement.
The Battle of Changshao between the states of Qi and Lu, though not large in scale, embodied valuable ancient military dialectical thinking in political strategy, grand strategy, and tactics, reflecting the basic principles of a weaker army fighting a stronger one. This battle is renowned in ancient Chinese military history for its defensive principle of striking only after the enemy has exhausted itself, achieving victory through a strategy of countering after being attacked.
The battles where the State of Lu interfered in the governance of the State of Qi and the State of Qi's invasion of Lu at the Battle of Changshao both took place between the states of Lu and Qi, yet their outcomes were vastly different, fully demonstrating that in wars of justice versus injustice, even under ancient conditions where only cold weapons were used, victory ultimately belongs to the righteous side.