What was the name of the Prince of Qin?
The Li family belonged to the northwest military aristocracy prevalent during the Sui dynasty and claimed to be paternally descended from the Daoist founder, Laozi (whose personal name was Li Dan or Li Er) the Han dynasty General Li Guang and Western Liang ruler Li Gao.  This family was known as the Longxi Li lineage (Li lineage; 隴西李氏), which includes the Tang poet Li Bai.  The Tang Emperors also had Xianbei maternal ancestry, from Emperor Gaozu of Tang's Xianbei mother, Duchess Dugu.
Li Yuan was Duke of Tang and governor of Taiyuan, modern Shanxi, during the Sui dynasty's collapse, which was caused in part by the Sui failure to conquer the northern part of the Korean peninsula during the Goguryeo–Sui War. He had prestige and military experience, and was a first cousin of Emperor Yang of Sui (their mothers were sisters). Li Yuan rose in rebellion in 617, along with his son and his equally militant daughter Princess Pingyang (d. 623), who raised and commanded her own troops. In winter 617, Li Yuan occupied Chang'an, relegated Emperor Yang to the position of Taishang Huang or retired emperor, and acted as regent to the puppet child-emperor, Yang You. On the news of Emperor Yang's murder by General Yuwen Huaji on June 18, 618, Li Yuan declared himself the emperor of a new dynasty, the Tang.Li Yuan, known as Emperor Gaozu of Tang, ruled until 626, when he was forcefully deposed by his son Li Shimin, the Prince of Qin.  Li Shimin had commanded troops since the age of 18, had prowess with bow and arrow, sword and lance and was known for his effective cavalry charges.  Fighting a numerically superior army, he defeated Dou Jiande (573–621) at Luoyang in the Battle of Hulao on May 28, 621.  In a violent elimination of royal family due to fear of assassination, Li Shimin ambushed and killed two of his brothers, Li Yuanji (b. 603) and Crown prince Li Jiancheng (b. 589), in the Xuanwu Gate Incident on July 2, 626.  Shortly thereafter, his father abdicated in his favor and Li Shimin ascended the throne.  He is conventionally known by his temple name Taizong.
Although killing two brothers and deposing his father contradicted the Confucian value of filial piety, Taizong showed himself to be a capable leader who listened to the advice of the wisest members of his council.  In 628, Emperor Taizong held a Buddhist memorial service for the casualties of war, and in 629 he had Buddhist monasteries erected at the sites of major battles so that monks could pray for the fallen on both sides of the fight.  This was during the Tang campaign against the Eastern Turks, a Turkic Khaganate that was destroyed after the capture of its ruler, Illig Qaghan by the famed Tang military officer Li Jing (571–649); who later became a Chancellor of the Tang dynasty. With this victory, the Turks accepted Taizong as their khagan, a title rendered as Tian Kehan in addition to his rule as emperor of China under the traditional title "Son of Heaven".
Li Shimin