During Zhu Youxiao's stay in Nanjing, the situation in South Vietnam changed dramatically, and the Zheng clan was forced to send envoys to ask for surrender in the face of the joint crusade of the Ming Dynasty with Siam, Chenla, and Champang. Zhu Youxiao knew that this was a consistent trick of South Vietnam, and it was in response to the saying that "there are no tigers in the mountains, and monkeys are kings." This time he finally mobilized the sea and land elites to go south, and he could take this opportunity to divide the resistance forces of South Vietnam and accomplish his set strategic goal, that is, to occupy the land north of the Red River, and to divide the south of the Red River into South Vietnam, Quang Nam, Hoa Anh, and Champang.

It turned out that Zheng Song of South Vietnam was old and about to die, and wanted to divide the power equally between Zheng Jie and Zheng Chun's two sons, Zheng Jie and Zheng Chun were very dissatisfied with this result, and secretly competed with each other. Zheng Song's younger brother Zheng Du took the opportunity to fan the flames, and soon Zheng Chun took the lead in launching a military coup d'état, ousting Zheng Song and seizing power alone.

The scheming Zheng Du superficially supported Zheng Chun, but secretly tricked Zheng Chun into his own military camp to kill him on the grounds that he needed Zheng's canonization. Zheng Du wanted to repeat his old tricks to deal with another nephew, but Zheng Jie was not fooled, and fled to Thanh Hoa overnight, holding the Li Dynasty Shenzong hostage to fight against his uncle. Zheng Du was originally a drunkard with a bad reputation, and he was quickly defeated in the face of Zheng Jie's offensive of "coercing the Son of Heaven to order the princes", and finally died in a foreign country.

During the internal strife in the Zheng clan, Yuan Chonghuan, Wang Sanshan, Yu Xuan and others took the opportunity to launch a counteroffensive, and soon repelled the Vietnamese army that invaded southern Yunnan. Zheng Jie, who had just taken control of South Vietnam at this time, was ambitious, and had suggested foreign expansion to his father Zheng Song in his early years, but now he had a heavy army and urgently needed a victory in a foreign war to establish his prestige, so he pointed the finger at the Ming army, which had no support. At the same time, he sent his minister Nguyen Wei Shi and internal supervisor Phan Van Thi to Quang Nam to demand that Nguyen Phuc Yuan, who was entrenched in Hue, hand over land endowments and strategic materials such as elephants, warships, and firearms.

Although Quang Viet submitted to the South Vietnamese regime, Nguyen Phuc Nguyen was also an ambitious politician who did not hesitate to marry several daughters to the Truong family as concubines in order to consolidate his position. After Nguyen Wei Shi and Phan Van Dice arrived in Hue, he first bribed the two with a lot of money to delay time, and at the same time sent the military general Nguyen Youyi to Thang Long in the name of mourning, and secretly contacted Zheng Jie's concubine Ruan Yuxiu, and obtained information that Zheng Ji was going to attack the Ming Dynasty.

Nguyen Phuc Nguyen wanted to inform the Ming Dynasty of this news immediately, but under the strong persuasion of the strategist Tao Weici, he made peace with the South Vietnamese on the one hand, and on the other hand, he hurriedly prepared for war, intending to adopt the strategy of sitting on the mountain and watching the tiger fight, so as to share the benefits in the war between the Ming Dynasty and the South Vietnam.

Contrary to Nguyen Phuc Yuan's strategy, Mo King Keng Kuan, who had been stationed in Cao Binh, saw this as a golden opportunity and personally led his army along the upper reaches of the Red River to invade Lao Cai and Jiaxing, and occupied a large area of land and population west of Jiaxing. After the victory in the first battle of Flash Dance, Mo Jingkuan was a little carried away by the victory, arrogantly thinking that the world was within reach, regardless of Yuan Chonghuan's dissuasion, he continued to lead troops to attack Taiyuan, wanting to take Shenglong in one fell swoop.

The cunning Zheng Jie saw at a glance the Achilles' heel of the Mo army's long battle line and poor support, first using the tactics of clearing the field in the front to thwart the spirit of the Mo army, and then led the troops around the rear of Mo Jingkuan to launch a surprise attack, and the Mo army immediately collapsed and was defeated on the whole front. Zheng Ji took the opportunity to march north to capture Gaoping, executed the emperor and crown prince of the Mo Dynasty, captured Mo Jingkuan's wife and concubines and children, and burned the Gaoping Palace.

After the defeat, Mo Jingkuan was like a lost dog, and surrendered to Yuan Chonghuan with his own soldiers and second son, saying that he was willing to take the Duke of Mu in Yunnan as an example, cancel the name of the year and the title of King Qing, and incorporate the land of the Mo Dynasty into the territory of the Ming Dynasty.

In the face of changes in the situation in South Vietnam, Zhu Youxiao adjusted his battle plan in a timely manner, first sending an envoy to ask Zheng Jie to withdraw from Gaoping and ceding the land north of the Red River to the Ming Dynasty, and at the same time ordering Zhu Kuiyuan, Sun Chuanting, and Huang Degong's 50,000 royal army to go north from Fenghua and attack Thang Long with Yuan Chonghuan of Lang Son on two fronts.

In Zhu Youxiao's view, generals can't take what they can't get with their swords, let alone expect diplomats to use their mouths, and the Ming Dynasty has withdrawn from South Vietnam since the Chengzu period, and there is no foundation for rule here, and it is difficult to effectively manage the forces of the Zheng and Li clans without eradicating them. What's more, Zheng Jie and his main army are in Gaoping, and the other part of the army was dragged by Yuan Chonghuan in Langshan, so when will it be better to take Thang Long in one fell swoop at this time? There is a cloud in the "Historical Records": "Heaven and Buddha take it, but they are to blame; When the time comes, you will suffer from it", how could Zhu Youxiao give up this opportunity.

Under the bombardment of the Ming artillery, the low wall of Thang Long soon collapsed into a huge gap, and Mo Jingkuan, who was at odds with the Zheng family, led his own soldiers to take the opportunity to charge, first following the example of Zheng Jie, brutally killing all the royal family members, and then beginning to slaughter the unarmed people. The massacre lasted for three days and three nights, and when Zhu Kuiyuan was about to enter the city to surrender, he found that Mo Jingkuan had not even left chickens and dogs, and the streets were full of blood, and there were corpses everywhere, and the air was filled with the smell of burning.

Seeing this tragic scene, Zhu Kuiyuan, who was accustomed to seeing big scenes, still felt a chill down his spine, and he understood that although Mo Jingkuan played the role of the executioner, the initiator was the little emperor. After the Ming army surrounded Thang Long, Zhu Youxiao ordered the Red River to be introduced into the city, which immediately crushed the Vietnamese army's will to resist. In the face of the enemy's request to surrender, Huang Degong and Sun Chuanting ignored it at all, and after the artillery concentrated its fire to blast open the city wall, Mo Jingkuan took the lead in entering the city, while the Ming army continued to snipe the fleeing enemy on the periphery.

After this battle, hundreds of thousands of South Vietnamese were killed and wounded, and the results of more than 200 years of painstaking management turned into ashes in an instant, and there was no longer the strength to compete with the Central Plains Dynasty, and Zheng Jie lost his backup supply, and the army mutinied in less than a month, and several generals did not hesitate to fight in order to invite merit, and finally Li Wenxiao took Zheng Jie's head to Taiyuan to ask for merit from Yuan Chonghuan, the defender of the Ming army.

Zhu Youxiao didn't want to know too much about the grievances between the Zheng family and the Li family, and the goal of this counterattack against the South Vietnamese invasion was to eliminate the power of these two families, so he decisively executed Li Wenxiao and others on the charge of killing the master, and publicly supported Mo Jingkuan, the commander of the light pole, as the commander of Annam, and established a South Vietnamese regime that was completely subordinate to the orders of the Ming Dynasty. The two sides were bounded by Fenghua, the mouth of the Red River, and the Ming Dynasty directly set up a state capital to the north, and Quang Binh to the south as the territory of South Vietnam. South Vietnam, with the Ming Dynasty as its suzerain, sent its only son, Mo Jingyu, to Beijing as a hostage.

In return, the Ming Dynasty recognized the status of the Mo Dynasty in South Vietnam as a vassal state, and handed over all of Le Van Xiao's troops to Mo Jingkuan's command, while providing free assistance with some obsolete firearms. Mo Jingkuan hugged the thigh of the Ming Dynasty and had no fear, asking Ruan Fuyuan of Quang Nam to pay tribute and send his son to serve the court.

Nguyen Phuc Nguyen originally wanted to gain some benefits in the war between the Ming Dynasty and the Zheng clan in South Vietnam, but he never thought that the Ming Dynasty would use the navy to go straight up the river from Phonghua and easily take Thang Long. At this time, he clearly realized the gap between himself and the behemoth of the Ming Dynasty, and in order to show his remorse, he decisively took Tao Weici's head as a vote of allegiance to the Ming Dynasty.