Emperor Taizong of Tang (28 January 598 – 10 July 649), previously Prince of Qin, personal name Li Shimin, was the second emperor of the Tang dynasty of China, ruling from 626 to 649. He is traditionally regarded as a co-founder of the dynasty for his role in encouraging his father Li Yuan (Emperor Gaozu) to rebel against the Sui dynasty at Jinyang in 617. Taizong subsequently played a pivotal role in defeating several of the dynasty's most dangerous opponents and solidifying its rule over China proper.
Taizong is considered to be one of the greatest emperors in China's history, and henceforth his reign became regarded as the exemplary model by which all future emperors were measured. His era, the "Reign of Zhenguan" (Chinese: 貞觀之治; pinyin: Zhēnguàn Zhī Zhì) is considered a golden age in ancient Chinese history and was treated as required studying material for future crown princes. Taizong continued to develop imperial examination systems. He asked his officials to be loyal to policies, not people, in order to eliminate corruption. Under the Zhenguan era, Tang China flourished economically and militarily. For more than a century after his death, China enjoyed prosperity and peace brought about by the solidification of imperial protection over the Chinese regions. In territorial extent, it covered most of the territories previously held by the Han dynasty as well as parts of modern-day Korea, Vietnam, Russia, Mongolia, Xinjiang, and Central Asia. This era of consolidation and conquest laid the foundation for Xuanzong's reign, which is considered to be the height of the Tang dynasty.
In 630, Emperor Taizong sent his general Li Jing against the Eastern Turks, defeating and capturing their Jiali Khan Ashina Duobi and destroying their power. This made Tang the dominant power in East and Central Asia, and Emperor Taizong subsequently took the title Khan of Heaven. He also launched a series of campaigns against the oasis states of the Tarim Basin, and against their main ally, the Western Turks. During his reign, Tang armies annexed Karakhoja in 640, Karasahr in 644, and Kucha in 648. Eventually, the Tang defeated and annexed the Western Turkic Khaganate after Su Dingfang defeated Qaghan Ashina Helu in 657.
Unlike much of the nobility in his time, Emperor Taizong was a frank rationalist and scholar of logic and scientific reason, openly scorning superstitions and claims of signs from the heavens. He also modified important rites in order to ease the burden of agricultural labour. The modern Chinese historian Bo Yang opined that Emperor Taizong achieved greatness by enduring criticism which others would find difficult to accept whilst trying hard not to abuse his absolute power (using Emperor Yang of Sui as a negative example), as well as by employing capable chancellors such as Fang Xuanling, Du Ruhui, and Wei Zheng. Emperor Taizong's wife Empress Zhangsun also proved to be a capable assistant.
Li Shimin was born in 598 at Wugong, in modern Xianyang, Shaanxi. His father Li Yuan, the duke of Tang, was a general of the Sui dynasty, and a nephew, by marriage, to Sui's founding emperor Emperor Wen. Li Shimin's grandmother Duchess Dugu was a sister of Empress Dugu, both of whom were daughters of Dugu Xin, a major Xianbei general during Sui's predecessor dynasty Northern Zhou. Li Shimin's mother, Li Yuan's wife Duchess Dou, was a daughter of Dou Yi (竇毅), the Duke of Shenwu, and his wife, Northern Zhou's Princess Xiangyang. Li Shimin was also of partial Xianbei descent. Duchess Dou bore Li Yuan four sons—an older brother to Li Shimin, Li Jiancheng, and two younger brothers, Li Yuanba (李元霸), who would die in 614, and Li Yuanji – and at least one daughter (the later Princess Pingyang). Li Yuan named Li Shimin "Shimin" as a shortened form of the phrase "save the earth and pacify the people" (濟世安民; jìshì ānmín). Li Shimin apparently showed talent early in his life, and in 613, the official Gao Shilian, impressed with him, gave him a niece (the later Empress Zhangsun) in marriage as his wife; he was 14 and she was 12. In 615, when Emperor Wen's son and successor Emperor Yang was ambushed by Eastern Turkic (Dongtujue) forces under Shibi Khan at Yanmen Commandery (present-day Daixian in Shanxi), a general call was made for men to join the army to help rescue the emperor. Li Shimin answered that call and served under the general Yun Dingxing (雲定興), apparently doing so with distinction. In 616, when Li Yuan was put in charge of the important city of Taiyuan, he brought Li Shimin with him to Taiyuan, while leaving at least three other sons – Li Jiancheng, Li Yuanji, and Li Zhiyun (李智雲, by Li Yuan's concubine Lady Wan) – at the ancestral home Hedong (河東, in modern Yuncheng, Shanxi).
Participation in the rebellion against Sui rule
Emperor Yang was soon dissatisfied with Li Yuan and Wang Rengong (王仁恭), the governor of Mayi Commandery (馬邑, roughly modern Shuozhou, Shanxi), over their inability to stop Eastern Turkic incursions and the growing strength of agrarian rebels, particularly the Eastern Turkic-supported Liu Wuzhou, who soon rose against Wang, killed him, and captured Emperor Yang's secondary palace near Taiyuan. Li Yuan also became fearful of a prophecy that the next emperor would be named Li—Emperor Yang had previously killed another official, Li Hun (李渾), and Li Hun's clan over his fear that Li Hun's nephew, Li Min (李敏), would seize the throne.
Fearful for his life, Li Yuan considered rebellion. However, he did not know that Li Shimin had also been secretly discussing plans for rebellion with Li Yuan's associates Pei Ji and Liu Wenjing. Once Li Shimin's plans matured, he had Pei inform Li Yuan of them—and also had Pei warn Li Yuan that if it were revealed that Li Yuan had had sexual relations with some of Emperor Yang's ladies in waiting at the secondary Jinyang Palace (晉陽宮, which Pei was in charge of and had allowed Li Yuan to do so), all of them would be slaughtered. Li Yuan agreed to rebel, and after secretly summoning Li Jiancheng and Li Yuanji from Hedong and his son-in-law Chai Shao (柴紹) from the capital Chang'an, he declared a rebellion, claiming to want to support Emperor Yang's grandson Yang You the Prince of Dai, nominally in charge at Chang'an with Emperor Yang at Jiangdu (江都, in modern Yangzhou, Jiangsu), as emperor. He made both Li Jiancheng and Li Shimin major generals and advanced southwest, toward Chang'an. He gave Li Shimin the title of Duke of Dunhuang. After defeating local Sui forces loyal to Emperor Yang, he defeated a Sui army of 30,000 men under the command of a veteran general of the wars in Korea outside of modern-day Beijing.
However, when Li Yuan arrived near Hedong, his army was bogged down by the weather. With food running out, there were rumors that the Eastern Turks and Liu Wuzhou would attack Taiyuan. Li Yuan initially ordered retreat, but at the earnest opposition by Li Jiancheng and Li Shimin, continued to advance. After defeating Sui forces at Huoyi (霍邑, also in modern Yuncheng), he decided to leave a small contingent to watch over Hedong while advancing across the Yellow River into Guanzhong (i.e., the Chang'an region). Once he did, he headed for Chang'an himself, while sending Li Jiancheng to capture the territory around the Tong Pass region to prevent Sui forces at Luoyang from reinforcing Chang'an and Li Shimin north of the Wei River to capture territory there. Meanwhile, Li Shimin's sister Pingyang had also risen in rebellion in support of him, and she was able to gather a sizeable army and capture some cities. She joined forces with Li Shimin and her husband Chai Shao. Soon, Li Yuan reconsolidated his forces and put Chang'an under siege. In winter 617, after defeating a large Sui army, he captured Chang'an from imperial forces and declared Yang You emperor (as Emperor Gong). He had himself made regent (with the title of grand chancellor), Li Shimin made the Duke of Qin, and created the Prince of Tang. Meanwhile, most of the Sui territories and armies did not recognize Emperor Gong as emperor, continuing to support the former Emperor Yang.