The Qin army marched thousands of li, passing through several feudal states to launch a surprise attack on the Zheng state, but the Jin state had already obtained intelligence. At that time, Duke Wen of Jin had just passed away, and Duke Xiang of Jin had recently ascended the throne. The Jin general Xian Zhen saw this as a golden opportunity to strike at Qin and advised Duke Xiang, saying that the Xiao Mountains were the Qin army's inevitable route, where dense forests, steep cliffs, and winding mountain paths allowed only a single war chariot to pass, making the terrain extremely perilous. They could use the natural danger of the Xiao Mountains to ambush the Qin forces. Duke Xiang of Jin heeded his advice, personally led a large army to the Xiao Mountains, ordered soldiers to cut down trees to block the path, planted a three-zhang-tall red Jin flag on top, and deployed troops in ambush around the area, with the signal to attack being the flag's fall. Then they waited for the Qin army to arrive.
The merchant Xian Gao falsely claimed to be an envoy from the state of Zheng, leading the Qin army to believe that Zheng was already on guard, forcing them to abandon their surprise attack plan. However, having marched a thousand li, they were unwilling to return without any gains, so they took the opportunity to destroy the small state of Hua, plundered a large amount of wealth, and then quickly led their troops back to Qin. When the Qin army reached Mianchi, Bai Yibing reminded Meng Mingshi that they were approaching Mount Xiao and must be cautious. But Meng Mingshi, confident in the Qin army's strength, believed no one would dare to ambush them. Thus, upon entering Mount Xiao, even when they saw a large Jin banner planted on a pile of wood blocking the road, he still did not take it seriously and ordered his soldiers to knock down the banner and clear the wood to open the path.
However, as soon as the red flag was lowered, the Jin army attacked from all directions, completely surrounding the Qin forces with no way to retreat. Forced back to an area piled with felled trees, which had been sprinkled with flammable materials like sulfur, the Qin troops were met with a barrage of flaming arrows as soon as they arrived. The trees instantly caught fire, turning the valley into a sea of flames. Three hundred chariots and several thousand soldiers were utterly annihilated, and the three Qin generals—Meng Mingshi, Xiqishu, and Bai Yibing—were all captured by the Jin state.
After Duke Xiang of Jin returned victorious to the court, he was overjoyed and planned to execute the three Qin generals to celebrate the victory. However, Duke Xiang's mother, Wen Ying, was the daughter of Duke Mu of Qin. Upon hearing that Meng Mingshi and the other two were captured, she did not want to create enmity with Qin. She said to Duke Xiang, "The states of Qin and Jin are originally relatives with a long history of good relations. Meng Mingshi and the others, in their pursuit of glory, have harmed the harmony between our two states. The ruler of Qin will surely resent them. But if you kill them, it may lead to deep hatred between the two states. It would be better to release them back, and the Qin ruler will certainly not let them off."
Hearing his mother's words, Duke Xiang of Jin felt she made sense, so he released the three prisoners, including Meng Mingshi. When Xian Zhen learned that Meng Mingshi had been freed, he rushed to see Duke Xiang, reproaching him for carelessly letting go of prisoners captured through the soldiers' bloody efforts. In his extreme anger, he disregarded proper conduct between ruler and subject and kept spitting repeatedly. Duke Xiang of Jin also regretted his decision and quickly sent General Yang Chufu with a troop to pursue them. After being released, Meng Mingshi and the other two feared the Duke of Jin would change his mind, so they fled at full speed. When they reached the Yellow River, Jin soldiers were already closing in. They hastily jumped onto a small boat by the riverbank and rowed desperately. By the time Yang Chufu arrived at the river, the boat had already left the shore. Without a boat, he shouted to them from the bank, "Our lord has prepared several fine horses to give you, so you can return to Qin as quickly as possible. Please come back and accept them."
Meng Mingshi knew it was a trap and naturally refused to fall for it. Standing at the bow of the ship, he bowed and said, "We are already deeply grateful for the generosity of the Lord of Jin and dare not accept any more gifts. If we manage to return alive, in three years' time we will surely come back to repay your state's great kindness!" And so, Meng Mingshi and his two companions narrowly escaped.
After Yang Chufu returned, he reported Meng Mingshi's words to Duke Xiang of Jin. Duke Xiang regretted his earlier decision, but since things had come to this, there was nothing he could do.
When Duke Mu of Qin learned that his entire Qin army had been annihilated, he deeply regretted not heeding Jian Shu's advice and stubbornly insisting on attacking Zheng. When Meng Mingshi and the other two returned to Qin, the duke, dressed in plain mourning clothes, personally went outside the city to the outskirts to welcome them. Meng Mingshi and the others quickly knelt down to beg for forgiveness, but Duke Mu of Qin said tearfully, "What fault is yours? It was I who did not listen to your father's words and caused you to lose the battle. I cannot overlook great achievements because of a small mistake." Not only did Duke Mu of Qin not blame them, but he also restored their official positions. Meng Mingshi and the others were deeply grateful and resolved never to forget the national disgrace, vowing to atone for their defeat with future merits.
From then on, they devoted themselves entirely to training troops and practicing battle formations, vowing to take revenge. In 626 BCE, Meng Mingshi, believing that the Qin army had become strong enough to defeat the Jin forces, requested permission from Duke Mu of Qin to go to war. Duke Mu then sent Meng Mingshi and others to lead the army, continuing the strategy of advancing eastward. Duke Hui of Jin, anticipating that Qin would not give up, had also been actively preparing for war. The Qin and Jin armies clashed at Pengya, fighting a fierce battle, and Qin was defeated once again. After two consecutive defeats, Qin's eastward advance was blocked, forcing them to turn westward for expansion.
Meng Mingshi originally wanted to avenge his defeat, but he lost another battle and thought that Duke Mu of Qin would surely not forgive him this time. However, to his surprise, Duke Mu still did not blame him and continued to let him command the army. Deeply grateful, Meng Mingshi began to reflect on the lessons of his past failures and examine himself. While improving his combat command skills, he sold all his property to comfort the families of fallen soldiers, personally trained the troops, and shared meals and hardships with them. At this time, Xian Jucu, the son of the Jin general Xian Zhen, was ordered by Duke Xiang of Jin to lead a coalition of troops from Jin, Song, Chen, and Zheng to attack Qin. The Qin army was in the midst of intense training, and after a calm analysis, Meng Mingshi believed that the coalition's morale was high while Qin was not yet prepared, so he ordered the city gates closed, refused to engage, and continued to intensify training. As a result, Jin captured two more cities of Qin. Many people thought that Meng Mingshi had become timid due to consecutive defeats and suggested that Duke Mu of Qin remove him from command, but Duke Mu still had full confidence in him.
By 625 BCE, the Qin army, trained by Meng Mingshi, had become strong and skilled, with formidable combat effectiveness. He felt it was time to attack the Jin army and avenge the defeat at Mount Xiao, so he requested Qin Duke Mu to personally lead the expedition. Duke Mu provided him with five hundred war chariots, along with well-equipped weapons and ample provisions, and also allocated grain and money to the families of the soldiers to ensure they had no worries behind them. After meticulous preparation, the troops were high-spirited and determined to compete with Jin. The three generals, Meng Mingshi, Xiqishu, and Bai Yibing, led the mighty Qin army, advancing toward Jin in a grand and imposing manner.
After the Qin army crossed the Yellow River, Meng Mingshi ordered all the ferry boats to be burned, signaling their determination to fight the Jin army to the death. Leading from the front as the vanguard, Meng Mingshi commanded the united and vengeful Qin forces, which swept through the enemy like a splitting bamboo, rampaging across Jin territory. Within days, they recaptured two cities previously seized by the Jin army, and then went on to conquer several more major Jin cities.
The Qin army was unstoppable, throwing the Jin court into panic. Jin Xianggong ordered the gates shut and the city defended, forbidding any engagement with the Qin forces.
Duke Mu of Qin, seeing that the lost territory had been recovered and that the state of Jin dared not come out of the city to engage in battle, having lost all its prestige, finally washed away the shame of the past. Realizing that a prolonged stalemate would benefit neither side, he led his army to Mount Xiao, where on the very battlefield of the previous year, he buried the bones of the fallen, wept in mourning for the slain soldiers, and delivered the famous "Oath of Qin" to honor their spirits. Meng Mingshi, Xiqishu, and Bai Yibing also knelt before the graves, weeping uncontrollably, and the entire army was deeply moved by the scene.