The Tang Dynasty’s Expansionist Vision

Emperor Taizong of Tang (r. 626–649) inherited an empire already primed for territorial expansion. The early Tang Dynasty (618–907) sought to consolidate China’s dominance over East Asia, particularly after the disastrous failures of the preceding Sui Dynasty’s Korean campaigns. Between 598 and 614, Emperor Yang of Sui had launched four catastrophic invasions of Goguryeo (Korea), draining imperial resources and contributing to the Sui collapse. Taizong, eager to restore Chinese prestige and secure the northeast, saw Goguryeo as both a strategic threat and a test of Tang military prowess.

Unlike the Sui, the Tang had stabilized internal governance and rebuilt a formidable army. Taizong, a seasoned military leader before his reign, believed a swift victory would deter Goguryeo’s ambitions and reinforce Tang hegemony. However, his 645 campaign—though initially successful—would become a cautionary tale of overreach.

The 645 Campaign: Triumphs and Stalemates

The Tang invasion began with decisive victories. Taizong’s forces captured the strategic city of Gaemo (盖牟城), seizing 20,000 prisoners and over 100,000 shi of grain. They then stormed Baiyan (白严), Liaodong (辽东), and a dozen other fortresses, demonstrating superior siege tactics. Yet the campaign’s turning point came at the siege of Anshi (安市城), where Goguryeo’s defenses held firm.

A critical opportunity emerged when Tang general Wang Daojian (王道宗) proposed a daring strike on Pyongyang, Goguryeo’s lightly defended capital:
“The entire Goguryeo army is concentrated here. With 5,000 elite troops, I could take Pyongyang, forcing their surrender.”

Taizong rejected the plan. His chancellor, Zhangsun Wuji (长孙无忌), argued that the emperor’s personal presence demanded caution. Historians speculate that Taizong, wary of overextending his supply lines, prioritized preserving his army over a risky decapitation strike.

By October, the Tang forces—bogged down at Anshi—faced Goguryeo reinforcements and a brutal Manchurian winter. Frostbite and starvation decimated the Tang cavalry, with 70–80% of horses dead. Taizong ordered a retreat, though not before inflicting 40,000 Goguryeo casualties at a cost of just 2,000 Tang soldiers.

The Mirror of Wei Zheng: A Lost Voice of Restraint

Taizong’s return journey was marked by introspection. He lamented the absence of his late advisor Wei Zheng (魏征), who had died in 643:
“Had Wei Zheng lived, he would have opposed this campaign.”

Wei Zheng had been Taizong’s “mirror”—a relentless critic who checked imperial excess. He had blocked Taizong’s controversial marriage to his brother’s widow and opposed the annexation of Gaochang (高昌), arguing its costs outweighed strategic benefits. Taizong had ignored the latter advice, seeing Gaochang as vital for Silk Road control. Yet the Korean campaign’s partial failure underscored Wei Zheng’s wisdom.

In a symbolic gesture, Taizong ordered Wei Zheng’s vandalized tomb restored—an admission of regret. He also honored fallen soldiers at Liucheng (柳城), weeping over his own elegy. These acts aimed to salvage morale, but the campaign’s geopolitical gains were negligible.

The Crown Prince Crisis: Domestic Fallout

The Korean expedition unfolded amid a destabilizing succession dispute. Taizong’s eldest son and heir, Li Chengqian (李承乾), had been deposed in 643 for plotting rebellion—a reaction to his father’s favoritism toward his brother Li Tai (李泰). Chengqian’s eccentricities (including adopting Turkic customs and keeping a male lover executed by Taizong) masked his desperation to secure his position.

With Chengqian exiled, Taizong favored Li Tai until his uncle Zhangsun Wuji intervened, warning that Li Tai might imitate Taizong’s own rise through fratricide (the 626 Xuanwu Gate Incident). The compromise candidate, Li Zhi (李治, later Emperor Gaozong), was pliable but weak—a choice reflecting Zhangsun’s desire for controllable leadership.

Taizong’s Korean failure thus intersected with dynastic uncertainty. His ragged battle robes, which he refused to replace until meeting Li Zhi, symbolized both paternal theatrics and deeper anxieties about his successor’s ability to uphold Tang power.

Legacy: The Costs of Imperial Overreach

Militarily, the 645 campaign proved Tang could punish Goguryeo but not conquer it. Taizong’s successor Gaozong would eventually defeat Goguryeo in 668 through protracted warfare and Silla’s alliance—a vindication of Wei Zheng’s preference for indirect pressure over direct invasion.

Culturally, the campaign reinforced Confucian ideals of ruler accountability. Taizong’s public remorse and rehabilitation of Wei Zheng became legendary, immortalized in Sima Guang’s Zizhi Tongjian as a model of imperial self-reflection.

Modern historians view the campaign as a pivot in Tang foreign policy. Subsequent emperors prioritized diplomacy over expansion, focusing on the Tibetan and Turkic threats. For Korea, the conflict entrenched resistance to Chinese domination, shaping a distinct national identity.

Ultimately, Taizong’s Korean venture—like Napoleon’s Russia or Hitler’s Stalingrad—stands as a lesson in the limits of power. Even brilliant strategists, when seduced by ambition, can misjudge geography, logistics, and the resilience of their foes.