In 1161, Emperor Wanyan Liang of the Jin Dynasty launched a massive invasion of the Southern Song Dynasty. When the Jin army reached Caishiji in Anhui, they clashed head-on with Southern Song troops and civilians led by the civil official Yu Yunwen. The previously aggressive Jin forces suffered a major defeat in this battle, and Wanyan Liang's plan to destroy the Southern Song was shattered. This battle, known as the Battle of Caishi, holds great significance in the history of the Song-Jin wars.
After the signing of the "Shaoxing Peace Treaty," the Song and Jin dynasties entered a prolonged period of stalemate. However, the insatiable Wanyan Liang was not satisfied with the Southern Song's submission to the Jin, including ceding territory and paying indemnities; he aimed to completely annihilate the Southern Song regime and seize the prosperous Jiangnan region for himself. Twelve years after ascending the throne, Wanyan Liang began to stir. He sent envoys to the Southern Song, demanding that its government cede the Huai-Han region to the Jin, and hurled abusive and highly provocative words at Emperor Gaozong.
This incident once again sparked debate between the pro-war and pro-surrender factions in the Southern Song court, with the pro-surrender faction advocating compromise and concessions to the Jin state, while the pro-war faction, led by Prime Minister Chen Kangbo, insisted on resisting the Jin. In the end, the voices of the pro-war faction prevailed over the pro-surrender faction, and the timid, risk-averse Emperor Gaozong could not go against the tide, so he ordered preparations to begin for the impending war.
In September 1161, Wanyan Liang personally led an army of 600,000 troops southward to attack. Facing the powerful Jin forces, the Song army was virtually unable to offer any resistance. The Jin army advanced unopposed, quickly reaching the northern bank of the Yangtze River and closing in on Lin'an, the capital of the Southern Song Dynasty.
The Southern Song court was in complete disarray, with Emperor Gaozong wanting to flee but being dissuaded by Chen Kangbo and others. At the same time, a military mutiny broke out within the Jin territory, and Wanyan Yong was installed as emperor. When news of this reached him, Wanyan Liang was furious; however, he did not immediately lead his army back to suppress the rebellion but instead continued his fierce assault on the Southern Song. After arriving at Caishi in Anhui, Wanyan Liang prepared to cross the Yangtze River from there and wipe out the Southern Song in one decisive move.
At that time, the Song army's commander Li Xianzhong had not yet arrived at Caishi, leaving the Song forces leaderless and their morale scattered. The civil official Yu Yunwen, in order to rally the Song soldiers and boost their spirits, said to them: "For now, the Yangtze River remains under our control. You must understand that the river's terrain is treacherous, easy to defend but hard to attack. Even if the Jin army possesses extraordinary skills, they will find it very difficult to cross successfully. As long as we hold the Yangtze River firmly and refuse to yield, a new turning point will surely come. As the saying goes, 'Soldiers are trained for a thousand days to be used in a single moment.' To protect our homes and defend our country, what does it matter if we must sacrifice our lives? But if the Jin army crosses the Yangtze and our nation falls, how could any of us continue to live on?"
Upon hearing these words, the soldiers immediately became invigorated. Yu Yunwen seized the opportunity to gather all the Song troops along the Yangtze River, ultimately assembling a total of 18,000 Song soldiers, who confronted 150,000 Jin troops head-on at Caishiji. Yu Yunwen was a civil official, not a military general, and commanding a battle was his first time, but he was not intimidated by this. Under his leadership and command, the Song soldiers ultimately succeeded in defeating the Jin army, which far outnumbered them.
After the defeat, Wanyan Liang hastily withdrew his troops and fled toward Yangzhou, while Yu Yunwen pursued him with the momentum of victory. Soon after, Wanyan Liang was killed by his own subordinates at Guazhou. Seizing this opportunity, the Song army recaptured the Huai River region that had been occupied by the Jin forces. With their leader gone, the Jin army lost the will to fight and quickly retreated back to the Jin state.
The Battle of Caishi preserved the Southern Song regime, sparing the people of Jiangnan from the trampling hooves of Jin cavalry. As the commander of this campaign, Yu Yunwen naturally made an indelible contribution. After the Jin army withdrew northward, both the Southern Song military and civilians praised Yu Yunwen profusely. Even today, Yu Yunwen's heroic deeds are still widely celebrated. Following the Battle of Caishi, Yu Yunwen, together with the Southern Song general Wu Lin, led a northern expedition, recovering six prefectures in Shaanxi and aiming to reclaim the entire Central Plains. However, Emperor Xiaozong, then on the throne, heeded the slander of petty men and ordered Yu Yunwen to withdraw, causing this northern campaign to ultimately fizzle out.
Later, Emperor Xiaozong once again ordered Yu Yunwen to lead an army to attack the Jin Dynasty, but Yu Yunwen happened to die of illness at that time. After Yu Yunwen's death, Emperor Xiaozong no longer had the heart to raise an army to attack the Jin Dynasty.
Emperor Shizong of Jin, Wanyan Yong, who was the reigning emperor of the Jin Dynasty at the time, was revered as the "Little Yao and Shun" for his virtuous governance. Under his leadership, the Jin military became even more formidable than before. Emperor Xiaozong of Song abandoned the Northern Expedition partly because he felt disheartened by the death of Yu Yunwen, and partly because the Southern Song's strength was indeed insufficient to resist the Jin Dynasty. During his reign, Emperor Xiaozong was diligent and incorruptible, bringing the Southern Song's national power to its peak. Unfortunately, he was born at the wrong time, coinciding with the era of Emperor Shizong of Jin. The tragedy of "Since there is Zhou Yu, why need Zhuge Liang?" played out again in Emperor Xiaozong's case. However, later generations did not deny his contributions, and he is widely recognized as the most accomplished emperor of the Southern Song Dynasty.