Emperor Gaozu of Tang

Emperor Gaozu of Tang, personal name Li Yuan, courtesy name Shude, Xianbei name Daye Yuan, was the founding emperor of the Tang dynasty of China, reigning from 618 to 626 CE. Under the Sui dynasty, Li Yuan was the governor in the area of modern-day Shanxi, and was based in Taiyuan.
In 615, Li Yuan was assigned to garrison Longxi. He gained much experience by dealing with the Göktürks of the north and was able to pacify them. Li Yuan was also able to gather support from these successes and, with the disintegration of the Sui dynasty in July 617, Li Yuan – urged on by his second son Li Shimin – rose in rebellion. Using the title of "Great Chancellor", Li Yuan installed a puppet child emperor, Yang You, but eventually removed him altogether and established the Tang dynasty in 618 with himself as emperor. His son and successor Li Shimin honoured him as Gaozu after his death.
Emperor Gaozu's reign was concentrated on uniting the empire under the Tang. Aided by Li Shimin, whom he created the Prince of Qin, he defeated all the other contenders, including Li Gui, Dou Jiande, Wang Shichong, Xue Rengao and Liu Wuzhou. By 628, the Tang dynasty had succeeded in uniting all of China. On the home front, he recognized the early successes forged by Emperor Wen of Sui and strove to emulate most of Emperor Wen's policies, including the equal distribution of land amongst his people, and he also lowered taxes. He abandoned the harsh system of law established by Emperor Yang of Sui as well as reforming the judicial system. These acts of reform paved the way for the reign of Emperor Taizong, which ultimately pushed Tang to the height of its power.
In 626, Li Shimin, in a dispute with his brothers Li Jiancheng, the Crown Prince, and Li Yuanji, the Prince of Qi, ambushed Li Jiancheng and Li Yuanji at Xuanwu Gate, killing them. Fearful of what Li Shimin might do next, Emperor Gaozu passed the throne to him and became Taishang Huang. He died on 25 June 635.

Early life and career

According to the official genealogy of the Tang ruling house, Li Yuan's seventh-generation ancestor was Li Gao, the Han Chinese founder of the Dunhuang-based Sixteen Kingdoms state of Western Liang. After Western Liang's destruction by the Northern Liang, Li Gao's grandson Li Zhong'Er served as a Northern Wei official, but for several generations after that, Li Yuan's ancestors had only minor military titles. Li Yuan's paternal grandfather Li Hu served as a major general under Western Wei's paramount general Yuwen Tai, and was created the Duke of Longxi and, as part of Yuwen's Xianbeification policy, given the Xianbei surname Daye. Li Hu died before Yuwen Tai's son Emperor Xiaomin of Northern Zhou founded Northern Zhou, but was posthumously created the Duke of Tang after Northern Zhou's founding. His son and Li Yuan's father Li Bing, of Han ethnicity, inherited the title of the Duke of Tang and married one of the Dugu sisters and daughter of the prominent general, Dugu Xin, who was a Xianbei of Xiongnu descent. Li Bing died in 572, and Li Yuan inherited the title of Duke of Tang, a title he continued to hold after the Northern Zhou throne was seized by Emperor Wen of Sui in 581, establishing the Sui dynasty, as Emperor Wen's wife, Empress Dugu, was an aunt of his. Around this time, Li Yuan's family also changed their surname from Daye back to Li.
At some point, Li Yuan married Lady Dou, a daughter of Dou Yi the Duke of Shenwu and Northern Zhou's Princess Xiangyang as his wife and duchess.
During Emperor Wen's reign, Li Yuan served three terms as a provincial governor. Early in the reign of Emperor Wen's son Emperor Yang, Li Yuan served as commandery governor, but was later recalled to serve as a junior minister within Emperor Yang's administration. When Emperor Yang carried out his second campaign against Goguryeo in 613, Li Yuan was in charge of part of the logistics operation. When the general Yang Xuangan rebelled near the eastern capital Luoyang, Emperor Yang commissioned Li Yuan as a general and made him be in charge of the operations west of the Tong Pass, although Yang Xuangan's rebellion eventually did not involve that region. Li Yuan took the opportunity to recruit talented people to his staff. Later that year, when Emperor Yang summoned him to his presence, he declined, citing ill health—an excuse that Emperor Yang did not believe, as he questioned Li Yuan's niece, Consort Wang, "Will he die?". In fear, Li Yuan took up drinking and receiving bribes to try to show Emperor Yang that he did not have great ambitions. In 615, Emperor Yang placed him in charge of the operations against agrarian rebels in the Hedong region, but recalled him in 616. Later that year, Emperor Yang put him in charge of the key city of Taiyuan.

Rebellion against Emperor Yang of Sui

Emperor Yang grew dissatisfied with Li Yuan and Wang Rengong, the governor of Mayi Commandery, over their inability to stop incursions by the Eastern Turks and the growing strength of agrarian rebels—particularly the Turk-supported Liu Wuzhou, the Dingyang Khan, who soon rose against Wang and killed him and captured Emperor Yang's secondary palace near Taiyuan. Li Yuan also became fearful due to prophecies circulating throughout the empire that the next emperor would be named Li—and because Emperor Yang had killed another official, Li Hun and his clan over his fears that Li Hun's nephew Li Min had imperial ambitions.
Traditional accounts, compiled during the reign of Li Yuan's second son by the Duchess Dou, Li Shimin, emphasize the latter's initiative and major role in instigating his father's rebellion. According to these, Li Shimin was secretly planning rebellion against Sui rule with Pei Ji, the majordomo of Emperor Yang's secondary palace, and with Liu Wenjing, the magistrate of Jinyang County, but at first did not reveal their plans to Li Yuan. At Li Shimin's urging, Pei Ji, who had also earlier, against regulations, allowed Li Yuan to have sexual relations with some of late Emperor Wen's imperial concubines, persuaded Li Yuan that it was necessary for him to rebel. Modern researchers, however, have concluded that the initiative for the revolt came from Li Yuan himself.
Li Yuan began to gather forces from the region, claiming that they were necessary to defend against the Turks, which drew suspicions from his deputies Wang Wei and Gao Junya. Li Yuan, afraid that Wang and Gao would act against him first, then used a Turkic attack as an excuse to falsely claim that Wang and Gao were working in concert with the Turkic khagan, Shibi Khan, and had them executed. He sent secret messengers to Hedong to recall his sons Li Jiancheng, Li Yuanji and Li Zhiyun, whom he had left there to watch over his household, and the capital Chang'an to recall his daughter and her husband Chai Shao. Li Jiancheng and Li Yuanji, leaving Li Zhiyun at Hedong, soon met with Chai, and they arrived together at Taiyuan. Li Yuan's daughter, believing it would be difficult for her to flee with Chai, chose to hide instead.
Once Li Jiancheng, Li Yuanji, and Chai arrived at Taiyuan, Li Yuan formally declared his rebellion, but maintained the guise of a Sui loyalist and declared that his intention was simply to install on the throne Emperor Yang's grandson Yang You, the Prince of Dai, who was then at Chang'an, and honor Emperor Yang as Taishang Huang. Li Yuan first secured his northern flank by contacting Shibi Khan, offering tribute, and received men and horses in exchange. He put Li Jiancheng and Li Shimin in charge of his army and, leaving Li Yuanji in charge at Taiyuan, advanced south. Meanwhile, the Sui officials at Hedong arrested Li Zhiyun and delivered him to Chang'an, where he was executed.
His daughter Pingyang sold her possessions to raise an army for him. She persuaded several other leaders to fight under her banner. They took several towns and her army swelled until she had 70,000 troops under her command.
Meanwhile, Li Yuan wrote another rebel leader, Li Mi the Duke of Wei, who was near Luoyang, trying to see if Li Mi would be willing to follow him, but Li Mi, believing in his own strength, had his secretary Zu Junyan write Li Yuan for him in this way:
Li Yuan was dismayed but, not wanting to make another enemy, wrote back humbly:
Li Mi was pleased with Li Yuan's response, believing that Li Yuan was willing to support him, and from that point on, Li Mi and Li Yuan often exchanged messengers. Li Yuan's campaign against Chang'an thus went without opposition from Li Mi. Meanwhile, however, when Li Yuan arrived near Hedong, his army was bogged down by the weather, and with food running out, there were rumors that Eastern Tujue 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, he decided to leave a small contingent to watch over Hedong while advancing across the Yellow River into Guanzhong. 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, his daughter had also risen in rebellion in support of him, and she was able to gather a sizable 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, he captured Chang'an and declared Yang You emperor. He had himself made regent and created the Prince of Tang. He sent his nephew Li Xiaogong south, and Li Xiaogong was able to persuade the Sui cities in modern southern Shaanxi, Sichuan, and Chongqing to submit.

Establishment of Tang and gradual unification

In spring 618, Emperor Yang was killed at Jiangdu in a coup led by the general Yuwen Huaji. When the news reached Chang'an, Li Yuan had Yang You yield the throne to him, becoming Emperor Gaozu of the Tang dynasty. He restored much of the institutions of Sui's first ruler, Emperor Wen, reversing a number of changes that Emperor Yang made. He created Yang You the Duke of Xi, Li Jiancheng, his oldest son, was named crown prince, while Li Shimin was made the Prince of Qin and Li Yuanji the Prince of Qi. Meanwhile, the Sui officials at Luoyang declared another grandson of Emperor Yang, Yang You'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. During the fall of 618, Xue took advantage of Li Shimin's illness to defeat an army commanded by Li Shimin and Liu Wenjing at Qianshui Plain and approach Chang'an. In response, Gaozu tried to enter an alliance with Li Gui, the Prince of Liang, between whose domain and the Tang Xue's Qin state was located, writing Li Gui and referring to him as cousin. Li Gui briefly submitted to 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, 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 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. 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 New Year 619, Li Mi requested Emperor Gaozu's permission to head east to persuade some of his former subordinates to submit to Tang, but once he left Chang'an, planned to restore his independence. He was ambushed and killed by the Tang general Sheng Yanshi.
In spring 619, Wang Shichong at Luoyang had Yang Tong yield the throne to him, ending the Sui dynasty 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. In summer 619, Li Gui's official An Xinggui, formerly a Tang official, rebelled against Li Gui and captured him, submitting to Tang. 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 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 the Eastern Turks. 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 the Prince of Huai'an, Emperor Gaozu's sister the Princess Tong'an, and Li Shiji's father Li Gai 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, 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 the Tang dynasty. Li Zitong, in turn, defeated and took over the territory of Shen Faxing the Prince of Liang, roughly modern Zhejiang.
After Li Shimin 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. The latter agreed, concerned that a Tang victory over Zheng would also mean his own demise, but at the same time was eager to exploit the weakness of the Zheng and claim its domains for himself. Emperor Gaozu was initially fearful that Dou and Wang would be able to sandwich Li Shimin's forces between them 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. Despairing, Wang also surrendered, and most of the Zheng territory was seized by the Tang. Xia territory was also seized by Tang, but after Emperor Gaozu executed Dou, Dou's general Liu Heita rose against the Tang and seized most of the former Xia territory, while Xu Yuanlang, a rebel leader who had previously submitted to Zheng, also rose in revolt, 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 as noted as Emperor Gao of Han's killing of Han Xin and Peng Yue, historians have 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, in 620, on accusation of treason.
Li Zhongwen the Duke of Zhenxiang, in 620, on accusation of collaboration with Eastern Tujue.
Liu Shirang the Duke of Yingyang, in 623, on accusation of collaboration with Eastern Tujue.