It's a question, it's kind of interesting, but it's also boring. Because all the answers are conjectures, it is difficult to say yin. Because the occurrence of all historical events is not accidental, it is the result of a long-term game in which various complex contradictions collide with each other, and it is also the external manifestation of human nature.
On the premise of not changing the personality and ability of the two but reversing the birth order, try to answer the following for Jun Yile:
First of all, if Li Shimin became the eldest son before Li Jiancheng, then he would have the natural inheritance rights granted to him by the patriarchal system.
In the process of Li Yuan's army from Jinyang to the occupation of Chang'an, Li Shimin's military talent will be vigorously displayed. The first battle in which he and Li Jiancheng captured Xihe together would establish a good image of commander in the army.
In this battle, Li Shimin did not have strictly trained soldiers and horses, and with the assistance of Li Jiancheng, Wen Dayou and others, he shared weal and woe with the soldiers all the way, strictly enforced military discipline, and took Xihe in only 9 days back and forth. made Li Yuan impressed with his son for the first time. Therefore, a strategy was formulated to enter Guanzhong.
Secondly, after Li Yuan became emperor, Li Shimin was naturally made the crown prince. The crown prince is the hope of the country's future and the foundation of the society, no matter how strong his military ability is, he will most likely stay in Beijing to assist Li Yuan in handling political affairs. and the second Li Jiancheng led the troops to fight.
However, Li Jiancheng's personality is benevolent, and he is not very good at running an espionage system. Therefore, although they will be wholeheartedly supported by all the officers and men in military operations, they will always lack the courage and determination at critical moments. As a result, the military process tends to slow down.
Liu Wuzhou, Song Jingang, Xue Ju and other separatist forces in the north were not much of a problem for Li Jiancheng. But Dou Jiande and Wang Shichong in the Kanto region will basically let Li Jiancheng return in vain.
After the defeat in the east, Li Yuan weighed the pros and cons and most likely made the ruthless Li Yuanji become Li Jiancheng's deputy. The two can complement each other well in personality. But it is also inevitable that quarrels and contradictions will arise.
Li Shimin, as the crown prince, was very anxious to watch the unification process of the Tang Dynasty, and would put forward strategic intentions and specific plans to Li Yuan according to his own judgment. After being verified, Li Shimin began to participate in military affairs in Beijing, which is also equivalent to Li Jiancheng who commanded by remote control. (Or start to reuse Li Jing, who has the merit of pacifying Jiangnan, then Li Jing's final ending is afraid that he will not die well.) In this way, Li Jiancheng's military achievements will also be greatly reduced, and his threat to Li Shimin will become the same level that Yang Guang did to Yang Yong. But it is not necessarily like Yang Guang replacing Yang Yong to replace Li Shimin. ）
With Li Shimin's participation, Li Jiancheng's military progress was very smooth, and Wang Shichong and Dou Jiande were eliminated. Li Yuan would find that Li Jiancheng, Li Yuanji and the others had rallied closely around him because of Li Shimin's calculations. The power of the crown prince is on the verge of surpassing the imperial power.
In the end, out of concern for the stability of the imperial power, Li Yuan began to try his best to balance the crown prince. So Li Jiancheng in his original position, not only received Shang Shuling, the Twelve Wei Generals, and Yongzhou Mu, but also gave birth to a Tiance general, and the most important thing is to allow this Tiance general to open a mansion. At the same time, he never mentioned Li Shimin's contributions.
Thanks to the convenience of opening the mansion, Li Jiancheng justifiably gathered all the civil ministers and military generals who followed him to fight the world in his Tiance General's Mansion. It is equivalent to having a complete set of legal succession teams.
When Li Shimin saw this scene, he knew that he was suspicious of his father. So he began to deliberately lower his posture, secretly enveloping the courtiers, and at the same time planting spies in various important positions, especially Li Jiancheng's Tiance General's Mansion. At the same time, he began to lure and co-opt Li Yuanji, and promised him to be the emperor's younger brother.
And Li Jiancheng's side, under the prodding of the government officials, and Li Yuan's intentional or unintentional hints, began to covet the position of the crown prince.
In this way, the three brothers started an open and secret fight, and the pattern created by Li Yuan in order to balance the prince and the prince was officially formed. Li Yuan looked at this kind of political situation where you have me, I have you, and he can finally sleep peacefully.
However, the ministers of the DPRK and China were at a loss and had to choose sides. Li Shimin and Li Jiancheng also began to take advantage of the emperor's non-stop beating of each other's small reports.
Although Li Yuan was annoyed, he couldn't do it every time. Gradually, brotherly rivalry became the consensus of the government and the opposition, and both sides regarded each other as enemies under the coercion of their respective forces.
Li Shimin also took advantage of the convenience of his status as the crown prince and began to disintegrate the forces that divided Li Jiancheng. In order to protect himself, Li Jiancheng, with the support of double agent Li Yuanji, decided to take a risk.
And Li Yuanji also fell to Li Shimin, and became the commander in chief at the pass that the Turkic Koubian needed to resist through the support of both sides and Li Shimin's covert support. At the same time, it took away Li Jiancheng's living force.
When both sides thought that Li Yuanji was their own, Li Yuanji saw an opportunity to reap the benefits.
So at the banquet practiced for Li Yuanji in Kunming Pool, Li Jiancheng staged a coup d'état, but because of Li Yuanji's ill intentions, he chose to wait and see. As a result, Li Jiancheng and Li Shimin, who were originally in control, were both defeated. At the critical moment when Li Yuanji wanted to clean up the mess and control the emperor and force Li Yuan to change himself to the crown prince, his camp suddenly switched. And the guards of Xuanwu Gate have long been replaced by Li Shimin's own people, Li Yuanji wanted to compete with the snipe and the clam for the fisherman's profit, but he was unexpectedly caught by Li Shimin's mantis and the cicada and the yellow finch behind.
After taking full control of the emperor, Li Jiancheng and Li Yuanji, Li Shimin forced Li Yuan to issue an edict to give Li Jiancheng to death on the grounds of rebellion, and Li Yuanji was deposed as a concubine and released to Qianzhou.
A few days later, Li Yuan gave way to Li Shimin, who was ...... for Tang Taizong
This is just a speculation, Li Shimin and Li Jiancheng reversed the order of birth, Li Shimin has the upper hand in legal theory but will fall behind in military achievements. However, according to Li Shimin's excellent intelligence ability, he will be able to predict the opponent's every move at a critical moment, as if he is a god, so that he can plan in advance and deal with it calmly, which will greatly increase his chances of winning.
There is also a possibility that we can refer to the ending of the dispute between Li Chengqian and Li Tai: the two sons were both abandoned, and the other princes were cheaper. But there is a premise that the emperor must have absolute control over the army.