After Huan Xuan usurped the throne, Liu Yu, a general of the Northern Garrison Army, led his troops in an attack on Jiankang. Unable to resist, Huan Xun fled with Emperor An of Jin. Later, Huan Xuan was killed, and Liu Yu welcomed Emperor An back to the throne. In reality, Emperor An was merely Liu Yu's puppet, as the actual power of the Eastern Jin Dynasty was completely in Liu Yu's hands.
Liu Yu came from a humble background and grew up in extreme poverty, forcing him to shoulder the burden of his family from a young age through hard physical labor. As he grew older, Liu Yu joined the military and repeatedly distinguished himself in battle, especially by restoring Emperor An of Jin to the throne, which brought him both fame and fortune. He not only seized real power over the Eastern Jin Dynasty but was also hailed as a national hero. However, this was far from enough for someone plotting to usurp the Eastern Jin throne. Because of his lowly origins, he lacked the support of the powerful aristocratic families. To expand his influence, enhance his prestige, and gain political capital for his bid to seize the imperial throne, he decided to launch a northern expedition.
In 409 AD, Liu Yu led a massive army from Jiankang, officially launching his northern expedition. His first target was the Southern Yan kingdom. The ruler of Southern Yan, Murong Chao, was a fatuous sovereign. Although he was quite intelligent, he had no interest in governing the state, only thinking about how to levy taxes on the people and where to go hunting. When Liu Yu's army advanced toward Southern Yan, Murong Chao panicked and had no idea how to respond. His ministers proposed three strategies: first, to offer no defense and, when Liu Yu's army attacked the city, send troops out to fight a decisive battle—this was the worst plan; second, to fortify the defenses and clear the fields, denying Liu Yu's army supplies, so that after a period of stalemate, Liu Yu would surely retreat—this was the moderate plan; third, to avoid the main force of Liu Yu's army, delay as much as possible, then send elite troops to cut off Liu Yu's supply lines and attack from both front and rear—this was the best plan. Murong Chao knew nothing about warfare and ultimately chose the worst plan, soon suffering a crushing defeat at the hands of Liu Yu.
Seeing that his kingdom was on the verge of collapse, Murong Chao immediately sent envoys to the neighboring Later Qin for help. Yao Xing, the ruler of Later Qin, fearing that Liu Yu would turn his forces against Later Qin after destroying Southern Yan, agreed to assist the former Yan. He dispatched an envoy to the Jin army camp to tell Liu Yu that Later Qin had already deployed 100,000 troops in Luoyang, and if Liu Yu attacked Southern Yan, Later Qin would definitely come to its aid.
After hearing this, Liu Yu contemptuously said to the envoy from Later Qin, "Go back and tell Yao Xing that I originally only planned to destroy Southern Yan, not your state. But if you insist on seeking your own doom, I will take care of you both at once, sparing myself the trouble of a long campaign later."
After the envoys of Later Qin departed, some worried that Liu Yu's words would anger Yao Xing and cause Later Qin to send troops. Liu Yu, however, said very calmly: "As the saying goes, 'Speed is the essence of war.' If Later Qin truly intended to attack, they would have done so in secret, with no need to notify us in advance. Their current actions are nothing more than a bluff. Right now, they are locked in battle with the Xia Kingdom—how could they possibly have the time to concern themselves with others?"
Liu Yu was right; at that time, the Later Qin was at war with the Xia state and had no time to attend to the Southern Yan. Shortly after, Liu Yu destroyed the isolated Southern Yan.
A few years later, Liu Yu led his troops north again, this time targeting the Later Qin. He entrusted state affairs to Liu Muzhi, the Left Deputy Director of the Imperial Secretariat, while he himself commanded the navy to advance north along the Yellow River. The vanguard, led by Tan Daoji and Wang Zhen'e, directed the infantry toward Luoyang.
The vanguard of the Jin army, led by Tan Daoji and Wang Zhen'e, captured Luoyang with great ease. Liu Yu originally planned to order the vanguard not to advance rashly after taking Luoyang, waiting instead for the main forces to arrive before pushing deeper into Later Qin territory. However, seeing that Later Qin was plagued by constant internal strife and that the defenses at Tong Pass were weak, Tan Daoji and Wang Zhen'e split their forces into two columns to attack Tong Pass. After encountering stubborn resistance from Later Qin troops, Tan Daoji and Wang Zhen'e found themselves locked in a stalemate with the enemy.
In 417 AD, Liu Yu personally led a large army northward. Yao Hong, the ruler of Later Qin, realizing he could not withstand Liu Yu's forces, sought aid from Emperor Tuoba Si of Northern Wei. At that time, Northern Wei was very powerful, and Tuoba Si decided to help Later Qin confront Liu Yu. He assembled 100,000 troops on the northern bank of the Yellow River, threatening Liu Yu's army.
Liu Yu ordered his navy to advance along the Yellow River by boat. Unexpectedly, Northern Wei dispatched several thousand cavalry to follow along the bank, continuously harassing the Jin army. At the time, the wind and waves were fierce, and many Jin warships were swept to the northern shore of the Yellow River, with soldiers falling into the water. The Northern Wei cavalry then seized the opportunity to kill the Jin troops. When the Jin soldiers went ashore to pursue them, the Northern Wei cavalry fled without a fight. In this way, the Jin army's officers and soldiers were thrown into confusion, not knowing what to do.
Liu Yu was extremely furious and wanted to deliver a heavy blow to the arrogance of the Northern Wei cavalry. After pondering for a long time, he finally came up with a brilliant idea. He called a general over and whispered a few words in his ear. Soon after, that general led a hundred chariots and seven hundred soldiers to the northern bank of the Yellow River, where they arranged a strange formation along the riverbank. This formation was semicircular, bulging in the middle, with both ends pressed against the riverbank. Two soldiers were hidden within the formation, and a white feather was planted high on one of the chariots in the center. Because this formation resembled a crescent moon, Liu Yu named it the "Crescent Moon Formation."
The Northern Wei cavalry had never seen such a strange formation, so they dared not act rashly. At that moment, someone raised a white feather from the chariot positioned in the center of the formation, and then two thousand soldiers climbed onto the chariots carrying one hundred large crossbows. The Northern Wei cavalry thought this formation was just a trick, so they charged together at the Jin army's position. The soldiers on the one hundred Jin chariots drew their crossbows and shot arrows, killing the Northern Wei cavalry at the front. However, the Northern Wei cavalry were too numerous, and these one hundred crossbows could not hold them off. Just as the Northern Wei cavalry surged like a tide toward the Jin army's position, the Jin army had already arranged extremely sharp spears, about three to four feet long, behind the Crescent Moon Formation, mounted on the large crossbows. The Northern Wei forces only focused on charging forward, and the Jin soldiers picked up large iron hammers, striking the crossbows with all their might. The spears on the crossbows flew like shooting stars toward the Northern Wei cavalry, instantly killing several thousand of them. The Northern Wei cavalry charging from behind, seeing the formation's deadly power, were terrified and fled in all directions. The Jin army pursued fiercely, killing many Northern Wei cavalry.
Liu Yu used the Crescent Moon Formation to deliver a heavy blow to the Northern Wei cavalry, clearing the westward route along the Yellow River and allowing his navy to advance smoothly. Subsequently, Tan Daoji and Wang Zhen'e led their infantry to join Liu Yu at Tong Pass, greatly boosting the Jin army's momentum. They soon captured Chang'an and annihilated the Later Qin state.
In 419 AD, Emperor An of Jin passed away, and Emperor Gong of Jin ascended the throne. Liu Yu, believing the time was ripe to claim the imperial title, forced Emperor Gong of Jin to abdicate in his favor. In 420 AD, Liu Yu declared himself emperor, changed the dynastic title to Song, and the Eastern Jin Dynasty officially came to an end.