Emperor Gaozu of Tang - Establishment of Tang and Gradual Unification

Establishment of Tang and Gradual Unification

In spring 618, Emperor Yang was killed at Jiangdu (江都, in modern Yangzhou, Jiangsu) in a coup led by the general Yuwen Huaji. When the news reached Chang'an, Li Yuan had Emperor Gong yield the throne to him, establishing Tang Dynasty as its Emperor Gaozu. He restored much of the institutions of Sui's Emperor Wen, reversing a number of changes that Emperor Yang made. He created Emperor Gong the Duke of Xi, and created Li Jiancheng, as his oldest son, crown prince, while creating Li Shimin the Prince of Qin and Li Yuanji the Prince of Qi. Meanwhile, the Sui officials at Luoyang declared another grandson of Emperor Yang, Emperor Gong's brother Yang Tong the Prince of Yue, as emperor, and refused to recognize the regime change in Chang'an.

Emperor Gaozu's rule immediately faced a major challenge from Xue Ju, an agrarian leader who had declared himself the Emperor of Qin, as Xue, during the fall of 618, took advantage of Li Shimin's illness to defeat an army commanded by Li Shimin and Liu Wenjing at Qianshui Plain (in modern Xianyang, Shanxi) and approach Chang'an. Emperor Gaozu, in response, tried to enter an alliance with Li Gui the Prince of Liang, between whose domain and the Tang was Xue's Qin state, writing Li Gui and referring to him as cousin. Li Gui briefly submitted to Emperor Gaozu. Meanwhile, before he could attack Chang'an, Xue Ju died of illness and was succeeded by his son Xue Rengao, who was a capable commander but who had alienated his generals because of his cruelty. Li Shimin was soon able to attack Xue Rengao at Gaozhi (高墌, in modern Xianyang as well), and force Xue Rengao to surrender.

Meanwhile, Li Mi, having been defeated earlier in the year in a surprise attack by the Sui general Wang Shichong, fled to Tang territory and submitted to Emperor Gaozu. Li Mi's general Xu Shiji, who controlled a major part of Li Mi's former territory, also submitted, and Emperor Gaozu, impressed with Xu's faithfulness to Li Mi, bestowed the imperial surname of Li on Xu. Emperor Gaozu created Li Mi the Duke of Xing, but only made him the Minister of Feasts, a post that Li Mi viewed as below his stature. Around the new year 619, Li Mi requested Emperor Gaozu's permission to head east to persuade some of his former subordinates to submit to Tang, and once he left Chang'an, planned to restore his independence. He was ambushed and killed by the Tang general Sheng Yanshi (盛彥師).

In spring 619, Wang had Yang Tong yield the throne to him, ending Sui and establishing a new state of Zheng.

Around the same time, Li Gui, while stating that he wished to be a Tang subject, refused the Tang creation of Prince of Liang, instead declaring himself the Emperor of Tang. Emperor Gaozu broke off his relations with Li Gui. In summer 619, Li Gui's official An Xinggui (安興貴), formerly a Tang official, rebelled against Li Gui and captured him, submitting to Tang. Emperor Gaozu executed Li Gui and incorporated his domain into Tang. Also around the same time, the rebel leader Du Fuwei, who controlled the modern southern Anhui, submitted to Tang, and Emperor Gaozu also bestowed the imperial surname of Li on him, creating him the Prince of Wu. Similarly, Luo Yi, who controlled the modern Beijing region, submitted, was bestowed the imperial surname of Li, and was created the Prince of Yan.

Meanwhile, Tang was facing another serious threat—Liu Wuzhou, now determined to march south against Tang. Emperor Gaozu sent Pei Ji against Liu's advancing army, but Pei was defeated by Liu, who then put Taiyuan under siege. Li Yuanji fled back to Chang'an, and much of modern Shanxi was seized by Liu. Emperor Gaozu then sent Li Shimin against Liu, and by summer 620, Li Shimin had defeated Liu, forcing him to flee to Eastern Tujue. Liu's territory was incorporated into Tang. Around the same time, however, Dou Jiande the Prince of Xia made a major offensive against the cities that had submitted to Tang in modern Hebei and Henan, north of the Yellow River, seizing nearly all of them and taking Emperor Gaozu's cousin Li Shentong (李神通) the Prince of Huai'an, Emperor Gaozu's sister the Princess Tong'an, and Li Shiji's father Li Gai (李蓋) captive. With Li Gai in Dou's custody, Li Shiji surrendered to Dou as well. In 620, Li Shiji, in association with another Tang general who surrendered to Dou, Li Shanghu (李商胡), plotted to ambush Dou, but the plot was discovered; Li Shanghu was killed, and Li Shiji fled back to Tang.

In 620, Li Fuwei captured much of the territory of another agrarian ruler, Li Zitong the Emperor of Wu, in the lower Yangtze River region, in the name of Tang Dynasty. Li Zitong, in turn, defeated and took over the territory of Shen Faxing the Prince of Liang, roughly modern Zhejiang.

After Li Shiimin defeated Liu, he started a campaign against Wang's Zheng state in fall 620. He initially could not decisively defeat Zheng, but by spring 621 had put the Zheng capital Luoyang under a tight siege, although he was not able to capture it. Wang sought aid from Dou, and Dou, believing that if Zheng were destroyed, his Xia state would be cornered, agreed to render aid. Emperor Gaozu was initially fearful that Dou and Wang would be able to sandwich Li Shimin's forces and ordered Li Shimin to retreat, but upon Li Shimin's petition changed his mind and permitted Li Shimin to remain in the Luoyang region. Li Shimin, leaving Li Yuanji in charge of the siege of Luoyang, advanced and took up position at Hulao Pass. In summer 621, the Tang and Xia forces engaged at Hulao, and Li Shimin defeated Dou, capturing him. In fear, Wang also surrendered. Most of Zheng territory was seized by Tang. Xia territory was also seized by Tang, but after Emperor Gaozu executed Dou, Dou's general Liu Heita rose against Tang, seizing most of former Xia territory, while Xu Yuanlang, a rebel leader who had previously submitted to Zheng, also rose, occupying the modern Shandong region.

Also in 621, Li Xiaogong defeated Xiao Xian the Emperor of Liang, who had controlled the modern Hubei, Hunan, and Guangxi region, forcing Xiao Xian to surrender. On another front, Li Fuwei's lieutenant Fu Gongshi defeated Li Zitong, forcing him to surrender as well. Liang and Wu territory were seized by Tang.

Meanwhile, while not noted as Emperor Gao of Han's killing of Han Xin and Peng Yue, the historians had nevertheless noted that some contributors to Emperor Gaozu's establishment of Tang were wrongly killed by him or killed based on fairly little evidence of wrongdoing:

• Liu Wenjing, in 619, on accusation that he engaged sorcerers.
• Emperor Gaozu's cousin Dugu Huai'en (獨孤懷恩), in 620, on accusation of treason.
• Li Zhongwen (李仲文) the Duke of Zhenxiang, in 620, on accusation of collaboration with Eastern Tujue.
• Liu Shirang (劉世讓) the Duke of Yingyang, in 623, on accusation of collaboration with Eastern Tujue.