On 9 July 642,[2] Li Jiancheng's title of Crown Prince Yin (隐太子) was restored.
He was the oldest son of Li Yuan, then Duke of Tang, who was a hereditary noble, and Li Yuan's wife Duchess Dou, who was the daughter of Dou Yi (竇毅), the Duke of Shenwu and Northern Zhou's Princess Xiangyang.
She also gave birth to a girl who eventually became Princess Pingyang, although it is not clear whether she was older or younger than Li Jiancheng.
[3] In 616, with rebellions beginning to engulf northern China and frequent Eastern Tujue incursions across the border, Emperor Yang of Sui commissioned Li Yuan to be the commander of Taiyuan, guarding it against both rebels and the Eastern Tujue.
As the chief commander of Taiyuan, Li Yuan was obligated by law to leave his family behind.
Another source, Wen Daya's (溫大雅) Da Tang Chuangye Qijuzhu (大唐創業起居注), attributes the decision to revolt to Li Yuan himself.
At the urging of Li Yuan's daughter, Chai Shao left Chang'an on his own while she went into hiding.
After he crossed the Yellow River into Guanzhong (i.e., the Chang'an region), he sent Li Jiancheng with Liu Wenjing (劉文靜) east to guard Tong Pass and Yongfeng Storage (永豐倉) and to stop any potential Sui reinforcements from the eastern capital Luoyang.
Later in the spring of 618, Emperor Yang, then at Jiangdu (江都, in modern Yangzhou, Jiangsu), was killed in a coup led by the general Yuwen Huaji.
Later that year Li Gui, the Emperor of Liang, was captured in a coup by his official An Xinggui (安興貴), who then surrendered to Tang.
In spring 621 the Xiongnu chieftain Liu Xiancheng (劉仚成) harassed Tang's border territory.
Although Li Jiancheng had made contributions towards Tang's reunification of China, Li Shimin had defeated a number of the most important contenders including Xue Rengao, the Emperor of Qin, Wang Shichong, the Emperor of Zheng, and Dou Jiande, the Prince of Xia.
By winter 622, the only remaining major threat against Tang rule was Liu Heita, the Prince of Handong.
He had been a Xia general, and rose against Tang after Li Yuan had executed Dou Jiande.
Around the turn of the year, Liu's forces were bogged down while attacking Tang's Wei Prefecture (魏州, in modern Handan, Hebei).
Liu fled north toward the Eastern Tujue, but was ambushed and captured by his own official Zhuge Dewei (諸葛德威), who delivered him to Li Jiancheng.
Li Jiancheng briefly flirted with the idea of occupying Chang'an instead of accepting the order, but eventually reported to Renzhi Palace to request forgiveness.
Li Shimin reported through officials at his residence that he started vomiting a lot of a blood after returning home.
They expressed their concerns to Li Yuan's ministers, who explained to him that this plan could escalate a brotherly rivalry into a civil conflict.
"Qijuzhu" is a type of imperial diary which records the emperor's daily activities that appeared as early as the Han dynasty.
He served as Li Yuan's staff of records and witnessed the establishment of Tang dynasty in 618.
Da Tang Chuangye Qijuzhu was a three-chapter book with a detailed narrative and a firsthand account of the 357 days of insurrection.
[14][15] Da Tang Chuangye Qijuzhu emphasizes that Li Yuan was the mastermind of the revolt against the Sui.
He knew how to recruit talented people and did well in his early political career as a crown prince.
Li Yuan, in Wen Daya's narrative, was a fearless leader with vision, courage and strategic planning.
Contemporary historians have re-examined the evidence for events contained in the official histories (the Old Book of Tang, the New Book of Tang and Zizhi Tongjian) and have concluded that important elements were probably fabricated during the reign of Li Shimin at his order.
On the basis of other material, most importantly Da-Tang Chuangye Qijuzhu, historians have been able to correct some of the bias and distortions of the official histories about the founding of the Tang dynasty.
[15][17][18][19][20] Da Tang Chuangye Qijuzhu provides a clear example of how Li Shimin rewritten history for his own favor.
The truth that had intentionally been distorted and ignored by later official historians can be found in the Da Tang Chuangye Qijuzhu.
[21][need quotation to verify] Consort and issue(s) Li Jiancheng's tombstone was lost for many years.