The change of Xuanwu Gate is a typical beheading behavior--the beheading is the head of the crown prince Li Jiancheng, not the head of emperor Li Yuan.
Li Jiancheng was killed, and Li Shimin was soon made crown prince and became emperor two months later. Why is Li Yuan willing to push the boat along the water? Could it be that Li Yuan was controlled by Li Shimin? Where did Li Yuan's forbidden army go?
Well, you can say that Li Yuan's forbidden army was rebelled by Li Shimin. Originally, an important reason why the change of Xuanwu Gate could succeed was that Li Shimin plotted against Chang He, the defender of Xuanwu Gate.
However, even if the forbidden army was rebelled and controlled by Li Shimin, what about the other armies of the Tang Dynasty? Couldn't Li Yuan control it too? Or did Li Shimin simply put his father under house arrest, cut off internal and external ties, and did not let him contact the army at all?
There is such a possibility. But I am more convinced that Li Shimin will not put Li Yuan under house arrest. Even if the father-son relationship is only a fig leaf in politics, Li Shimin would rather have this fig leaf - because Li Yuan, who lost Li Jiancheng, is no threat to Li Shimin.
No, I am not saying that Li Yuan's army was commanded by Li Jiancheng, and after Li Jiancheng's death, Li Yuan could not move. But as the emperor of feudal society, Li Yuan could only choose a successor from his son, and there was no other possibility.
In the year of the Xuanwumen Incident, Li Yuan was sixty years old. However, with the exception of Jiancheng, Shimin, Yuanji, and Xuanba, who died early (that is, Li Yuanba, the world's best man in the Sui and Tang Dynasties), Li Yuan's other sons were still young.
Li Yuan's fifth son, Li Zhiyun, was born in 603 and died at the age of sixteen, before the Tang Dynasty was established. The sixth son, Li Yuanjing, was born in 618, the year of the founding of the Tang Dynasty—he was an eight-year-old child by 628, the year of the Xuanwumen Revolution.
Sixty-year-old Li Yuan, do you still have the time and energy to train an eight-year-old child to become the ruler of the empire? He didn't.
Therefore, as soon as Li Yuan heard that Jiancheng Yuanji was dead, he immediately ordered the army to be transferred to Li Shimin's command, not only surrendering, but also surrendering very cooperatively--the "General History of China" and "Zizhi Tongjian" both recorded Li Yuan's words: "This is my long-cherished heart", as if he had long wanted to pass the throne to Li Shimin.
It is not that Li Yuan is weak and can be deceived, but that the situation is stronger than the will - if Li Yuan pursues, punishes, and kills Li Shimin, he has no adult son, he will bring the Tang Dynasty to the dilemma of "the lord is less suspicious of the country".
He wouldn't have taken such a risk.