Emperor Taizong of Tang

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 against 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 of Tengeri Qaghan. 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.