Taizong subsequently played a pivotal role in defeating several of the dynasty's most dangerous opponents and solidifying its rule over China proper.

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.

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.

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.

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.

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).

Xue Rengao was forced to withdraw into the city of Gaozhi (高墌, in modern Xianyang as well), and once he did, his soldiers began surrendering to Li Shimin en masse.

Around new year 619, Emperor Gaozu made Li Shimin Taiwei (太尉, one of the Three Excellencies) and put him in charge of Tang operations east of the Tong Pass.

Dou would not do so, and, against the advice of his wife Empress Cao and secretary general Ling Jing (凌敬) that he should instead attack Tang's prefectures in modern southern Shanxi, he marched toward Luoyang.

In the winter of 621, the Xia general Liu Heita rose against Tang rule, claiming to be avenging Dou, whom Emperor Gaozu had executed against Li Shimin's wishes after he had been brought back to Chang'an.

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.

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.

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.

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 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.

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).

Also by 642, Xueyantuo had posed a sufficiently serious threat (albeit still formally submissive) that Emperor Taizong saw two alternatives—destroying it by force or forming into a heqin relationship by marrying one of his daughters to Yinan.

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][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.

[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).

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.

Taizong removed him from the position of heir apparent and chose as his successor Li Zhi, who embraced Han Chinese cultural heritage.

By summer 645, Tang forces had captured Yodong Fortress (遼東, in modern Liaoyang, Liaoning), and headed southeast toward the Goguryeo capital Pyongyang.

[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.

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.

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.

[41] After consolidating the Tang Empire, Emperor Taizong ordered six large stone panels, known as Zhaoling Liujun (昭陵六駿), to be carved with the portraits of his favorite horses.

Since I engaged in military campaigns, those war chargers which carried me rushing on the enemy and breaking the line, and which rescued me from perils, their true images should be portrayed on stone and be placed left and right of my tomb to demonstrate the righteousness of "curtain and cover.

Constructed between 636 and 649 CE, the stone reliefs "exemplify the beauty of early Tang sculpture; the images are realistic yet powerful, created with simplicity and matured craftsmanship.

"[43] After Taizong's death, the monuments were situated along the east and west sides of the "spirit path" at the North Gate of the tomb complex named Zhaoling or Zhao Mausoleum.