Li Yi's Rebellion

Also in 627, the general Li Yi the Prince of Yan—a late-Sui warlord who later submitted to Tang, who associated with Li Jiancheng—fearing that Emperor Taizong would eventually take action against him, rebelled at Bin Prefecture (豳州, in modern Xianyang), but was quickly crushed by the official Yang Ji (楊岌) and killed in flight. Later that year, when Emperor Gaozu's cousin Li Youliang (李幼良) the Prince of Changle, the commandant at Liang Prefecture (涼州, roughly modern Wuwei, Gansu), was accused of allowing his staff to oppress the people and to trade with Qiang and Xiongnu tribesmen, Emperor Taizong sent the chancellor Yuwen Shiji (Yuwen Huaji's brother) to investigate, and in fear, Li Youliang's staff members plotted to hold him hostage and rebel. When this was discovered, Emperor Taizong forced Li Youliang to commit suicide. Late in the year, Wang Junkuo (王君廓), the commandant at You Prefecture (幽州, roughly modern Beijing), also rebelled, but was defeated quickly and killed in flight. However, although there were also reports that Feng Ang (馮盎), a warlord in the Lingnan region, was rebelling, Emperor Taizong, at Wei's suggestion, sent messengers to comfort Feng, and Feng submitted.

Also in 627, Emperor Taizong, seeing that there were too many prefectures and counties, consolidated and merged many of them, and further created another level of local political organization above prefectures—the circuit (道; dào)—dividing his state into 10 circuits.

Campaign against the Eastern Turks

Emperor Taizong depicted giving an audience to Gar Tongtsen Yulsung, the ambassador of the Tibetan Empire, in a later copy of a painting by court artist Yan Liben (600–673 AD)

In 628, with Ashina Duobi and Ashina Shibobi having a falling out, Ashina Shibobi submitted to Emperor Taizong, as did the chieftains of Khitan tribes, who had previously submitted to the Eastern Turks. With their khaganate in turmoil, Ashina Duobi was no longer able to protect the last late-Sui rebel ruler who alone remained standing against Tang pressure—Liang Shidu the Emperor of Liang, and in summer 628, with the Tang generals Chai Shao and Xue Wanjun (薛萬均) sieging the Liang capital Shuofang (in modern Yulin, Shaanxi), Liang Shidu's cousin Liang Luoren (梁洛仁) killed Liang Shidu and surrendered, finally uniting China. With the Eastern Turkic Khaganate weakened, their vassal Xueyantuo also broke away and formed its own khanate, and Emperor Taizong entered into an alliance with Xueyantuo's leader Yinan, creating Yinan the Zhenzhupiqie Khan (or Zhenzhu Khan in short).

In late 629, believing the time ripe for a major attack on the Eastern Turks, Emperor Taizong commissioned the general Li Jing with overall command of a multi-pronged army, assisted by the generals Li Shiji, Li Daozong, Chai Shao, Xue Wanche (薛萬徹, Xue Wanjun's brother) and Su Dingfang, attacking the Eastern Turks at multiple points. The army was successful in its attacks, forcing Ashina Duobi to flee, and by late spring 630, Ashina Duobi had been captured, and Eastern-Turkic chieftains all submitted to Tang. Emperor Taizong spared Ashina Duobi but detained him at Chang'an, and he considered what to do with the Eastern Turks.

The main opposing views were from the chancellors Wen Yanbo (who advocated leaving the Turks within China's borders to serve as a defense perimeter) and Wei (who advocated leaving them outside the borders). Emperor Taizong accepted Wen's suggestion and established a number of prefectures to accommodate the Turks, leaving them governed by their chieftains without creating a new khan to govern them.

In 631, Emperor Taizong established a feudal scheme, where the contributors to his reign were given, in addition to their current posts, additional posts as prefectural governors, to be passed on to their descendants. Soon, however, receiving much opposition to the plan, the strongest of which came from Zhangsun Wuji, Emperor Taizong cancelled the scheme. After the conquest of the Eastern Turkic Khaganate, Emperor Taizong's officials repeatedly requested that he carry out sacrifices to heaven and earth at Mount Tai, and Emperor Taizong, while at times tempted by the proposal, was repeatedly dissuaded from doing so by Wei, who pointed out the expenses and the labors that would be imposed on the people as a result, and also that this would open China's borders to attack.

Campaign against Tuyuhun

Around this time, Tang was having increasing conflicts with Tuyuhun, whose Busabo Khan Murong Fuyun, under instigation by his strategist the Prince of Tianzhu, had been repeatedly attacking Tang prefectures on the borders. At one point, Murong Fuyun sought to have a Tang princess marry his son the Prince of Zun, but the marriage negotiations broke down over Emperor Taizong's insistence that the Prince of Zun come to Chang'an for the wedding. In summer 634, Emperor Taizong had the generals Duan Zhixuan and Fan Xing (樊興) lead forces against Tuyuhun, but with Tuyuhun's forces highly mobile and avoiding direct confrontation, Duan, while not defeated, could not make major gains. Once Duan withdrew, Tuyuhun resumed hostilities. In winter 634, with the Tibetan king Songtsän Gampo making overtures to marry a Tang princess as well, Emperor Taizong sent the emissary Feng Dexia (馮德遐) to Tibet with an eye toward an alliance against Tuyuhun. In winter 634, he commissioned Li Jing, assisted by the other generals Hou Junji, Xue Wanjun, Xue Wanche, Qibi Heli, Li Daozong, Li Daliang, Li Daoyan (李道彥), and Gao Zengsheng (高甑生), to attack Tuyuhun. In 635, Li Jing's forces crushed Tuyuhun forces. Murong Fuyun was killed by his own subordinates, and his son Murong Shun killed the Prince of Tianzhu and surrendered. Emperor Taizong created Murong Shun the new khan, although Murong Shun was soon assassinated. Emperor Taizong then created Murong Shun's son Murong Nuohebo as the new khan.

Also in 635, Emperor Gaozu died, and Emperor Taizong, observing a mourning period, briefly had Li Chengqian serve as regent, and after he resumed his authorities less than two months later, he still authorized Li Chengqian to thereafter rule on minor matters.

In spring 636, Emperor Taizong commissioned his brothers and sons as commandants and changed their titles in accordance with the commands that they received, sending them to their posts—with the exception of his son Li Tai the Prince of Wei, who by this point was beginning to be highly favored by him. He further allowed Li Tai to engage literary men to serve as his assistants, as Li Tai favored literature. From this point on, Li Tai would be so favored that there began to be talks that Emperor Taizong might let him displace Li Chengqian, whose favors began to wane. In fall 636, Empress Zhangsun died. Emperor Taizong mourned her bitterly and personally wrote the text of her monument.

In summer 637, Emperor Taizong recreated the feudal scheme that he had considered and abandoned in 631, creating 35 hereditary prefect posts. (By 639, however, the system was again abandoned after much opposition.)

Treatment of Noble Clans

Fountain Memory, calligraphy of Emperor Taizong on a Tang stele.

Sometime before 638, Emperor Taizong, disgusted with the traditional noble clans of Cui, Lu, Li, and Zheng and believing that they were abusing their highly honored names, commissioned Gao Shilian, Wei Ting, Linghu Defen, and Cen Wenben to compile a work later to be known as the Records of Clans (氏族志), with the intent of dividing the clans into nine classes based on their past contributions, good deeds, and ill deeds. In an initial draft that Gao submitted, he nevertheless ranked the branch of the Cui clan that the official Cui Min'gan (崔民幹) belonged to as the highest, a decision that Emperor Taizong rebuked, as he pointed out that Gao was merely again looking at tradition and not the recent contributions. He therefore personally intervened in revising the work, reducing Cui's clan to the third class.

Tibetan attack on Songzhou

In fall 638, Tibet's Songtsän Gampo, displeased that Emperor Taizong had declined to give him a Tang princess in marriage and believing that Murong Nuohebo had persuaded Emperor Taizong to decline the marriage proposal, launched a major attack with forces of 200,000 on Tuyuhun and then on several Tang prefectures, putting Song Prefecture (松州, roughly modern Ngawa Tibetan and Qiang Autonomous Prefecture, Sichuan) under siege. Emperor Taizong commissioned Hou Junji, assisted by Zhishi Sili (執失思力), Niu Jinda (牛進達), and Liu Jian (劉簡), of a total of force of 50,000 to counterattack, and Niu, who commanded the forward forces, defeated Tibetan forces at Song Prefecture. Songsän Gampo withdrew and sued for peace, but still sought to marry a Tang princess. Emperor Taizong agreed this time. Also in 638, believing that Xueyantuo was growing increasingly strong and difficult to control, Emperor Taizong granted Yinan's sons Bazhuo and Jialibi (頡利苾) both lesser khan titles, to try to create dissensions between them. In summer 639, Ashina Jiesheshuai (阿史那結社率), the younger brother of Ashina Shibobi, whom Emperor Taizong did not favor and gave little recognition to, formed a conspiracy with Ashina Shibobi's son Ashina Hexiangu (阿史那賀暹鶻) to assassinate Emperor Taizong. They had planned to wait for Li Zhi the Prince of Jin to depart from the palace in the morning and use that opportunity to attack the palace. On the day they planned, however, Li Zhi did not leave the palace, and Ashina Jiesheshuai attacked anyway but was quickly defeated, captured, and executed. After this incident, however, the officials began advocating sending the Turks away from the heart of the state. In fall 639, Emperor Taizong created a Turkish prince who had served him faithfully, Li Simo (né Ashina Simo) as the khan of a newly recreated Eastern Turkic state (as Qilibi Khan), giving him all of the Turks and Xiongnu who had surrendered as his subordinates, to be settled north of the Great Wall and the Yellow River. However, the Turks were fearful of Xueyantuo and initially refused to head to their new location. Emperor Taizong issued an edict to Yinan that he and Li Simo keep their peace and not attack each other, and after receiving from Yinan the assurance that he would not attack, the Turks advanced to the new location.

Campaign against Western Turks

Taizong began a series of campaigns against the Western Turks (Xitujue) and their allies, the oasis states of the Tarim Basin, around 640.^[22] These hostilities between the Western Turks and Tang would continue until the defeat and conquest of the Western Turks in 657 under Taizong's successor, Gaozong.^[23] The kingdoms of Shule and Khotan surrendered to the Chinese in 632, as did the kingdom of Yarkand in 635.^[10] Qu Wentai (麴文泰), the king of Gaochang, who had previously been submissive to Tang, had become increasingly hostile to Tang, allying with the Western Turks. In 640, Emperor Taizong commissioned Hou Junji, assisted by Xue Wanjun, to launch a major attack on Gaochang. As they approached Gaochang, Qu Wentai died in fear and was succeeded by his son Qu Zhisheng (麴智盛). Qu Zhisheng offered to submit, but Hou demanded a surrender, which Qu Zhisheng refused. However, Hou put Gaochang under siege, and with aid from the Western Turks not arriving, Qu Zhisheng surrendered. Wei Zheng suggested that Emperor Taizong allow Qu Zhisheng to remain king, pointing out that the monetary and human costs would be high to keep a permanent garrison at Gaochang, but Emperor Taizong disagreed, and he converted Gaochang into two prefectures and annexed it into his state.

In winter 640, Songsän Gampo sent his prime minister Gar Tongtsen Yülsung ("Lu Dongzan" (祿東贊) in Chinese) as an emissary to Tang, offering tributes and again requesting marriage. Emperor Taizong created a daughter of a clansman as the Princess Wencheng, and in 641 sent Li Daozong to accompany Princess Wencheng to Tibet to preside over the wedding. In winter 641, believing that Emperor Taizong was about to carry out sacrifices to heaven and earth at Mount Tai and would be unable to aid the Eastern Turks, Yinan launched a major attack on them, commanded by his son Dadu (大度). Li Simo was forced to retreat inside the Great Wall. Emperor Taizong commissioned Li Shiji, assisted by Zhang Jian (張儉), Li Daliang, Zhang Shigui (張士貴), and Li Xiyu (李襲譽), to attack Xueyantuo. Li Shiji soon defeated Dadu at Nuozhen River (諾真水, flowing through modern Baotou, Inner Mongolia), and Dadu fled.

Contacts with the Byzantine Empire

Fanciful modern representation of the Byzantine embassy to Tang Taizong in 643 CE.

According to the Xi'an Stele, Emperor Taizong recognized the Nestorian Church of the East, due to efforts of the Christian missionary Alopen in 635 CE.^[25]

The Old Book of Tang and New Book of Tang mention several embassies made by Fu lin (拂菻; i.e. the Byzantine Empire), which they equated with Daqin (which may refer to the Roman Empire or Middle East), beginning in 643 with an embassy sent by the king Boduoli (波多力, i.e. Constans II Pogonatos) to Emperor Taizong,^[26] bearing gifts such as red glass and green gemstones.^[27] These histories also provided cursory descriptions of Constantinople and its walls,^[28] as well as how it was besieged by Da shi (大食; the Arabs of the Umayyad Caliphate) and their commander "Mo-yi" (摩拽; i.e. Muawiyah I, governor of Syria before becoming caliph), who forced the Byzantines to pay tribute.^[27][29] Henry Yule highlights the fact that Yazdegerd III (r. 632–651), last ruler of the Sasanian Empire, sent diplomats to China for securing aid from Emperor Taizong (considered the suzerain over Ferghana in Central Asia) during the loss of the Persian heartland to the Islamic Rashidun Caliphate, which may have also prompted the Byzantines to send envoys to China amid their recent loss of Syria to the Muslims.^[30] Tang Chinese sources also recorded how Sassanid prince Peroz III (636–679) fled to Tang China following the conquest of Persia by the growing Islamic caliphate.^[31]

The expansion of China's power into Central Asia under Emperor Taizong seems to have been noticed in the West. Theophylact Simocatta, a Byzantine Greek historian during the reign of Heraclius (r. 610–641), wrote that Taugast (or Taugas; Old Turkic: Tabghach, from Tuoba, the Xianbei clan of Northern Wei),^[32] was a great eastern empire in the Far East that ruled over Turkic people, with a capital city roughly 1,500 miles northeast from India that he called Khubdan (from the Turkic word Khumdan, meaning, Chang'an), where idolatry was practiced but the people were wise and lived by just laws.^[33] He depicted the Chinese empire as being divided by a great river (i.e. the Yangzi) that served as the boundary between two rival states at war, yet during the reign of Byzantine Emperor Maurice (582–602 AD) the northerners wearing "black coats" conquered the "red coats" of the south (black being a distinctive color worn by the people of Shaanxi, location of the Sui capital Sui Chang'an, according to the 16th-century Persian traveler Hajji Mahomed, or Chaggi Memet).^[34] This account may correspond to the conquest of the Chen dynasty and reunification of China by Emperor Wen of Sui (r. 581–604).^[34] Simocatta names their ruler as Taisson, which he claimed meant Son of God, either correlating to the Chinese Tianzi (i.e. Son of Heaven) or even the name of the contemporary ruler Emperor Taizong.^[35]

Succession dispute

Also in 643, Emperor Taizong would see major turmoil among his own closest family. In spring 643, his son Li You (李祐) the Prince of Qi, angry over restrictions that his secretary general Quan Wanji (權萬紀) had often placed on him, killed Quan and declared a rebellion. Emperor Taizong sent Li Shiji against Li You, but before Li Shiji could engage Li You, Li You was captured by his own subordinate Du Xingmin (杜行敏) and delivered to Chang'an, where Emperor Taizong ordered him to commit suicide and executed 44 of his associates.

The death of Li You drew out news of another plot. Li Chengqian, who had been fearful that Emperor Taizong would eventually remove him and replace him with Li Tai, had begun to conspire with Hou Junji, Li Yuanchang (李元昌) the Prince of Han (Emperor Taizong's brother), the general Li Anyan (李安儼), and his brothers-in-law Zhao Jie (趙節) and Du He (杜荷, Du Ruhui's son) to overthrow Emperor Taizong. During the investigations in the aftermaths of Li You's rebellion, one of the co-conspirators, Li Chengqian's guard Gegan Chengji (紇干承基), was implicated by association, and in order to save himself, he revealed Li Chengqian's plot. Emperor Taizong was shocked by the news, and he appointed Zhangsun Wuji, Fang Xuanling, Xiao Yu, and Li Shiji, along with the officials in charge of the supreme court and the legislative and examination bureaus of the government to carry out a joint investigation. At the suggestion of the mid-level official Lai Ji, Emperor Taizong deposed, but did not kill, Li Chengqian, while ordering Li Yuanchang to commit suicide and executing Hou Junji, Li Anyan, Zhao, and Du.

Taizong's eldest son Li Chengqian chose to speak Turkic, dress in Turkic clothes and even set up a tent in his residence in imitation of a Turkic khan. Taizong removed him from the position of heir apparent and chose as his successor Li Zhi, who embraced Han Chinese cultural heritage.^[36]

After Li Chengqian was deposed, Emperor Taizong briefly promised Li Tai that he would be made crown prince. However, as the investigations continued, Emperor Taizong came to the belief that Li Chengqian's downfall was driven by Li Tai's machinations, and therefore resolved to depose Li Tai as well. At Zhangsun's suggestion, Emperor Taizong created a younger son, Li Zhi the Prince of Jin (who, like Li Chengqian and Li Tai, were born of Empress Zhangsun), crown prince, who was considered kinder and gentler, while exiling Li Chengqian and Li Tai. However, starting later that year, Emperor Taizong began to doubt whether Li Zhi's personality was sufficiently strong to serve as emperor, and he toyed with the idea of making another son, Li Ke the Prince of Wu, a son of his concubine Consort Yang (Emperor Yang of Sui's daughter), crown prince, but did not do so due to strong opposition by Zhangsun Wuji.

Meanwhile, coming to the belief that he made an ill-advised promise to Yinan to give him Princess Xinxing in marriage, Emperor Taizong demanded a large amount of bride price – 50,000 horses, 10,000 cows and camels, and 100,000 sheep—a price that Yinan agreed to, but could not immediately collect and deliver. Emperor Taizong used it as an excuse to cancel the marriage agreement. Meanwhile, as Wei Zheng had, prior to his death, recommended Hou Junji and Li Chengqian's staff member Du Zhenglun as chancellors, Emperor Taizong came to suspect that Wei was part of the plot as well. He destroyed the monument he had authored for Wei and cancelled the betrothal between Wei Shuyu and Princess Hengshan.

Campaign against the Karasahr

In 644, with Yanqi's king Long Tuqizhi (龍突騎支), who had assisted the Tang campaign to conquer Gaochang, turning against Tang and allying with the Western Turks, Emperor Taizong sent the general Guo Xiaoke (郭孝恪), the commandant at Anxi (安西, i.e., Gaochang) to launch a surprise attack on Yanqi. Guo caught Long Tuqizhi by surprise and captured him, making his brother Long Lipozhun (龍栗婆準) regent. (The Western Turkic viceroy Ashina Quli (阿史那屈利) subsequently captured Long Lipozhun and briefly occupied Yanqi, although he then, not wanting a direct confrontation with Tang, withdrew, and the Yanqi nobles made Long Tuqizhi's cousin Long Xuepoanazhi (龍薛婆阿那支) king.)

Goguryeo-Tang War

The Sui dynasty tried to invade Goguryeo in 598, 612, 613 & 614. Taizong campaign (map) was in 645. Gaozong's campaigns were in 661, 667 & 668.

Also in 644, with Goguryeo attacking Silla and Silla requesting aid, Emperor Taizong decided to prepare for a campaign to conquer Goguryeo. He arrested the emissaries that Yeon sent to the Tang court, accusing them of disloyalty to King Yeongnyu. By winter 644, the mobilization was in full force. (Apparently because of Tang's preparation to attack Goguryeo, however, the reconstituted Eastern Turkic people, fearing an attack from Xueyantuo at a time that Tang would be ill-equipped to assist, panicked and abandoned their khan Li Simo, fleeing into Tang territory. Emperor Taizong reabsorbed the Eastern Turks into Tang, while making Li Simo a general in his army.)

In spring 645, Emperor Taizong departed from Luoyang and led the troops northeast, behind a vanguard of 60,000 commanded by Li Shiji and Li Daozong. At the same time, Zhang Liang led the other 40,000 from sea. By summer 645, Tang forces had captured Yodong Fortress (遼東, in modern Liaoyang, Liaoning), and headed southeast toward the Goguryeo capital Pyongyang. Emperor Taizong personally led about 30,000 Tang and tribal forces and defeated a 150,000 force commanded by two Gogureyo generals and then put Ansi (安市, in modern Anshan, Liaoning) fortress under siege.^[37]

However, the capable defense put up by Ansi's commanding general stymied Tang forces and, in late fall, after suffering some casualties, with winter fast approaching and his food supplies running out, Emperor Taizong withdrew. He much regretted launching the campaign and made the comment, "If Wei Zheng were still alive, he would never have let me launch this campaign." He reerected the monument he authored for Wei and summoned Wei's wife and children to meet him, treating them well.

Campaign against Xueyantuo and conquest of the Kucha

Meanwhile, in the aftermaths of the Goguryeo campaign, Xueyantuo's Duomi Khan Bazhuo (son of Yinan, who had died earlier in 645) launched attacks against Tang's border prefectures, with largely inconclusive results. In spring 646, the Tang generals Qiao Shiwang (喬師望) and Zhishi Sili counterattacked, defeating Bazhuo's forces, causing him to flee. His vassals Huige, Pugu (僕骨), and Tongluo (同羅) tribes took the opportunity to rebel and attack him. Hearing this, Emperor Taizong launched a major attack, commanded by Li Daozong, Ashina She'er (阿史那社爾), Zhishi Sili, Qibi Heli, Xue Wanche, and Zhang Jian, against Xueyantuo. With Xueyantuo under attack from multiple sides, Bazhuo was killed by Huige forces, and the remaining Xueyantuo people fled and supported Bazhuo's cousin Duomozhi as Yitewushi Khan, but soon offered to submit to Tang. Emperor Taizong sent Li Shiji toward Duomozhi's location, with the direction to either accept his submission or destroy him. Duomozhi surrendered and was taken to Chang'an, ending Xueyantuo's rule over the region. The other tribes formerly submissive to Xueyantuo offered Emperor Taizong the title of "Heavenly Khan" and thereafter largely became submissive to Tang.^{[lower-alpha 4][39]} Tang nominally established seven command posts and six prefectures over the region. (Huige's khan Yaoluoge Tumidu (藥羅葛吐迷度), while submissive to Tang, for some time tried to take control over the region himself, but was subsequently assassinated in 648, and there would be no other organized attempt by Huige to take over the region until for about another century.)

A bas-relief of a soldier and horse with elaborate saddle and stirrups, from the tomb of Emperor Taizong, c. 650. The relief shown here depicts "Autumn Dew," also known as "Whirlwind Victory" and is housed at the Penn Museum in Philadelphia, PA.

After the victory over Xueyantuo, Emperor Taizong again turned his attention toward to Goguryeo, cutting off relations once more and considering another campaign. Under suggestions by some of his officials, he decided to launch harassment campaigns against Goguryeo's northern region on a yearly basis, to weaken Goguryeo gradually. The first of these campaigns was launched in spring 647, with Li Shiji and Niu Jinda in command, and would reoccur. All this was in preparation of another campaign in 649 with forces totaling 300,000, but Taizong died before this campaign and the campaign was stalled into Gaozong's reign.

In 648, Emperor Taizong launched another campaign, commanded by Ashina She'er, aimed at Qiuzi, but first attacking Yanqi and killing Long Xuepoanazhi and replacing him with his cousin Long Xiannazhun (龍先那準). Ashina She'er advanced on Qiuzi and captured its king Bai Helibushibi (白訶黎布失畢), making his brother king instead.