Xu Yuanlang and Du Fuwei suddenly surrendered to the Xia State, which not only shocked the major forces in the Sui Empire, but also greatly stimulated the ambitious Li Tang regime.

After the Li Tang regime was strongly hindered from the expansion of Li Liangsuo's troops in the middle and lower reaches of the Yangtze River east of Yiling and north of Guanzhong, in order to increase his strength as much as possible, Li Yuan began to set his sights on Wang Shichong, who occupied the land of Heluo in the Central Plains.

Because Li Tang was supported by most of the people in the Guanlong Xianbei military aristocratic group, Li Yuan formed a relatively obvious military advantage over Wang Shichong.

He decided to no longer use the small-scale tug-of-war between one city and one place in the past, and prepared to launch a full-scale offensive against Wang Shichong, as far as possible to eliminate the Zheng regime established by Wang Shichong in a short time.

Wang Shichong, who had just stabilized the territory he had taken from the Wagang army, was shocked when he learned the news, after all, the territory he had snatched from the Wagang army was not stable, and in order to resist the Tang army, he selected 200,000 brave men from his counties to gather in Luoyang, and set up four town generals to guard the four cities.

In July of the 55th year of the Xia calendar, Li Shimin was ordered to lead an army to fight Wang Shichong.

Because Wang Shichong occupied Hongnong County and Hangu Pass and it was difficult to attack, Li Shimin first sent a part of the army from Tongguan to the east, making a posture of attacking Hongnong County with the main force, and then he personally led the army from Hedong County to suddenly cross the river south and attack the back of Hongnong County to seize the unsuspecting Hangu Pass.

Then and the Tang army out of Tongguan before and after the attack took the chaos of Hongnong County in one fell swoop, the Tang army's attack speed is so fast, the defenders of Heyin, Henan and other cities west of Luoyang actually did not know that the Tang army had attacked, and the city was conquered by the Tang army one after another, so that the Tang army attacked outside Luoyang in one breath.

Li Shimin's rapid attack shocked Wang Shichong, between the rapid offensive and strategic superiority of the Tang army, Wang Shichong's Zheng regime a large number of official troops have surrendered to the enemy, at this time Wang Shichong can no longer believe anyone, he only believes in his own family.

Therefore, Wang Shichong sent Wang Honglie, the king of Wei, to guard Xiangcheng County, King Wang Xingben to guard Hugaoguan (now northwest of Bishui County, Henan), Wang Tai, the king of Song, to guard Huaizhou (now Qinyang County, Henan), Wang Shiyun, the king of Qi, to guard Nancheng (outside the south gate of the imperial city), Wang Shiwei, the king of Chu, to guard Baocheng (imperial city), Prince Wang Xuanying to guard Dongcheng (east of the imperial city), Wang Xuanshu, king of Han, to guard Hanjia City, and King Lu Wang Daoyi to guard Yicheng (east of Dongcheng).

The generals appointed by Wang Shichong to lead the troops to defend one side were his own clan relatives, and he himself personally led the most elite 30,000 soldiers outside the city to resist the Tang army.

Xue Meng, the vanguard general of the Tang Army, led the front army to surround Cijian (now the east of Xin'an County, Henan), and Wang Shichong led 30,000 troops to come to the rescue in person.

However, at this time, Li Shimin led more than 30,000 cavalry to suddenly march into Cijian with the intention of encircling and annihilating it, and Wang Shichong had to abandon Cijian and retreat to Luoyang.

Subsequently, Li Shimin sent Shi Wanbao south from Yiyang to occupy Longmen (southwest of present-day Yichuan County), Liu Weide to surround Hanoi (present-day Qinyang County, Henan) from Taihang East (present-day Jiyuan County, Shanxi), and Wang Junkuo to cut off Shichong's road for transporting military rations from Luokou (present-day southeast of Gongxian County, Henan).

Huang Junhan (note, this is the jailer who let Zhai Rang go in the first place) went from Heyang to attack Luocheng (now southeast of Mengjin County, Henan), and Li Shimin personally led a large army to camp in Beibishan and forced Luoyang City, which Wang Shichong personally defended.

In August, Huang Junhan sent the captain Zhang Yesha to lead the army to attack and recapture Luocheng, cutting off the Heyang South Bridge, Wang Shichong was furious and sent the prince Xuanying and the general Yang Gongqing to attack Huiluo, but failed to conquer it, so he had to build a castle in the west of Huiluocheng and leave troops to defend it.

Soon, Liu Dewei conquered the outer city of Huaizhou City (present-day Qinyang County, Henan), and the situation became more and more unfavorable for Wang Shichong.

In September, Wang Shichong's troops surrendered the two counties of Runan and Huai'an, the governor of Xianzhou (now Qinyang County, Henan), to Li Tang, so Li Tang began to bypass Luoyang City to attack Huaiyang, Qiaojun, Ruyin and other Huaibei counties.

In the face of the deteriorating situation, the generals under Wang Shichong were full of pessimism, so on September 10, Zhang Zhenzhou, a general under Wang Shichong, suddenly surrendered to Li Tang, and Xue Meng took advantage of the situation to capture Kip Stone Fort (now west of Luoyang County, Henan) and Qianjin Fort (now north of Luoyang County, Henan).

Immediately, Wang Shichong's general Yang Qing surrendered to the Tang Dynasty with Luyang County (now Lushan, Henan), so the Tang army began to attack Zhongchuan and Rubei counties from Nanyang County to the north, and the Tang army had threatened the safety of Luoyang City from both directions.

Wang Shichong's crown prince Wang Yingxuan guarded the tiger prison, and the army was stationed between Xingshui and Bianshui, and when he learned that Luyang County had descended to the Tang Dynasty, he immediately led his troops to seize it, but was blocked by the Tang army.

Subsequently, Xingzhou and Bianzhou successively descended to the Tang Dynasty, and the Tang army opened up the passage between the south of Songshan and the Tang army in Heyin.

Faced with the deteriorating situation of the Tang army's continuous siege of Luoyang and the gradual occupation of his own territory by the Tang, Wang Shichong had to decide to send someone to Dou Jiande to ask for assistance from Dou Jiande, who was north of the Great River, after all, Li Yuan was the only remaining separatist force.

Although Wang Shichong and Dou Jiande had clashed over some territories such as Hanoi in the past, Liu Bin, Jiande's Chinese scholar, believed that he should seize the opportunity to temporarily abandon his previous suspicions and send troops to help.

He analyzed: The world is in turmoil, Tang has Guanxi, Zheng has Henan, and Zhao has Hebei, and they have become a full trend. Today, the Tang Dynasty raised troops to Zheng, since autumn and winter. Tang soldiers are increasing day by day, Zheng Di is declining, Tang is strong and Zheng is weak, and it is bound to be lost. If Zheng dies, Zhao can't be independent, so it's better to resolve revenge and send troops to save him. Zhao attacked the outside, Zheng attacked the inside, and the Tang Dynasty must be broken. The Tang division has retreated, Xu Guan has changed, if Zheng can take it, and the soldiers of the two countries, take advantage of the old Tang division, the world can also be desirable.

Liu Bin's analysis of the situation can be described as to the point, killing two birds with one stone, and it is feasible. At that time, the Tang army was strong, and neither Zhao nor Zheng alone was enough to confront the Tang army, and only by maintaining a three-legged stand could it survive, and Dou Jiande adopted his advice.

In March of the 56th year of the Xia calendar, Dou Jiande ordered Fan Yuan to defend Caozhou, Shan Xiongxin and other generals led the people to the west to rescue Luoyang, and the land and water were combined to transport grain with a fleet, and the garrison was stationed in Dongyuan east of Chenggao.

At this time, the Tang army besieged Luoyang, even with the help of gunpowder weapons, because the Luoyang city wall was five zhang thick, and the inferior gunpowder made by Li Tang could not blow it up at all.

At this time, in the face of Dou Jiande's coming, most people thought that 'my soldiers are tired and old, and the world is full of defending the city, and it is not easy to pull out, and Jiande comes from the ground, and he is sharp. I suffered from the enemy on my back, and I advocated retreating from Xin'an to accept its shortcomings.

However, the general Guo Xiaoke opposed the withdrawal of troops, and he believed that 'the world is full of poverty, the generals are facing Fu, and Jiande is far away to help him, and this heaven intends to kill both.' It is advisable to reject it according to the danger of the prison, and to move from time to time, and it must be broken! ’

Xue Shui also agreed with this opinion, and further supplemented Guo Xiaoke's opinion, now the Li Tang regime is facing not only Dou Jiande and Wang Shichong, but also the most powerful Xia State in the future, and only by unifying the heavens as soon as possible can it maintain a certain resistance in the future war with the Xia State.

Li Shimin agreed with Guo Xiaoke and Xue Shui's analysis and suggestions, and in response to the opposition of Xiao Yu and other generals, he clearly decided to continue fighting with Dou Jiande's army.

So he divided his command and ordered Li Yuanji, the king of Qi, to besiege Luoyang, and he personally led 3,500 elite soldiers to the east into the tiger prison.

After arriving at the Tiger Prison, Shimin led people to reconnoiter Dou Jiande's military situation, and ambushed 500 horsemen along the way, and only took four brave generals such as Captain Chi Jingde to follow.

When a few people were found by the other party at a distance of three miles from Dou Jiande's military camp, General Dou Jiande sent more than 5,000 cavalry to pursue, Li Shimin and others quickly retreated and led them into the ambush circle, and the surrounding Tang army defeated the enemy army and beheaded more than 300 levels, which greatly consolidated the morale of the Tang army.

Jiande led his troops to the west, was blocked by the Tang army in the Tiger Prison Pass for nearly a month, several battles and were lost, the soldiers began to think about returning under successive setbacks, and the Tang army sent light troops to intercept his military rations, constantly disturbing Dou Jiande's rear grain road.

Faced with this situation, he convened his soldiers to discuss the strategy of using troops, and Ling Jing persuaded King Dou Jiande to first capture Huaizhou and Heyang north of the river, send generals to garrison, and then cross the Taihang Mountains to attack Shangdang, and quickly seize Li Tang's Fen, Jin and other places in the hometown of Fenhe in Hedong, and then seize Pujin to threaten Guanzhong.

Ling Jing's strategy was also very good, but Wang Shichong repeatedly sent people to Jiande for help and begged to urge him to send troops, and the generals under Dou Jiande also felt that their troops far exceeded the Tang army, and they all advocated a decisive battle with the Tang army with the intention of underestimating the enemy.

Although Jiande's wife Cao also advised him to follow Ling Jing's advice, Jiande ignored it and still decided to lead his army to confront Tang Zuo head-on.

After some reconnaissance, Dou Jiande planned to attack the Tiger Pass when the Tang army ran out of horses and released them in Hebei.

In May, Li Shimin crossed the river in the north and Guangwu in the south, deliberately letting his horse go to the river to lure the other party, and then he himself rushed back to the Tiger Prison that night.

The next day, Dou Jiande of the middle plan really led the whole army to attack the Tiger Prison Pass, Li Shimin did not come out of the wall, Dou Jiande waited for two hours, and the soldiers sat down to rest under hunger, because the weather was hot and they were fighting to drink water and were undisciplined.

At this moment, Li Shimin took advantage of the dispersal of Dou Jiande's army to lead the crowd into battle and rushed straight into its formation.

And Dou Jiande was meeting the ministers at this time, the Tang cavalry suddenly killed, the ministers all ran to Dou Jiande, Dou Jiande ordered the cavalry to fight, and ordered the ministers who were blocking the way to retreat, just between this advance and retreat, the Tang army had already killed in front of him, and Dou Jiande had to retreat to the eastern slope.

At this time, the two armies fought fiercely, and Li Shimin personally led several generals to roll up the Tang Army battle flag and rush to the back of Dou Jiande's army formation, and then opened the flag.

Dou Jiande's army, which was fighting fiercely, thought that his rear army had collapsed, so the whole army was defeated, and Dou Jiande was also captured in the war.