The Tongjian on the Tang Dynasty Glory and Decline

The Tang dynasty (618 to 907 CE) is often called the golden age of Chinese civilization. Its poetry, painting, and cosmopolitan culture set standards that later dynasties could aspire to but never fully match. The Zizhi Tongjian covers the Tang in extraordinary detail, devoting roughly a third of its total length to these 289 years. Sima Guang was deliberate about this. The Tang offered the richest material for his central project: demonstrating how greatness is built, how it is maintained, and how it is lost. For the full scope of what the Tongjian covers, see What Zizhi Tongjian Covers.

The Founding: Li Yuan and His Sons

The Tang was founded by Li Yuan, a Sui general who rebelled when the Sui dynasty was already collapsing. The Tongjian records the founding with its usual precision: the counsel Li Yuan received from his sons, the strategic timing of his march from Taiyuan to the capital, and the rapid transfer of loyalty among Sui officials. The actual military campaign was brief because the Sui state had already disintegrated. What Sima Guang emphasizes is the institutional inheritance. Li Yuan did not build a new state from nothing. He took over the Sui’s administrative structure, its legal code, its examination system, and its military organization. For the Sui background, see Sima Guang and the Making of the Comprehensive Mirror.

Li Yuan’s second son, Li Shimin (later Emperor Taizong), is one of the Tongjian‘s most carefully studied figures. The succession was violent. Li Shimin killed his brother, the designated heir, at the Xuanwu Gate in 626 CE and effectively forced his father to abdicate. The Tongjian records the event without softening, but it also records what followed: twenty-three years of governance that became the model for all subsequent Chinese rulers.

Emperor Taizong: The Zhenguan Era

The Zhenguan era (627 to 649 CE) is the Tongjian‘s gold standard for governance. Sima Guang devotes extensive space to Taizong’s reign because it demonstrates every principle he wants to illustrate about how a ruler should behave. Taizong was not a perfect human being. He killed his brother and pressured his father. But as a ruler, he cultivated a quality that the Tongjian values above all others: the willingness to listen to criticism.

The most famous relationship in the Tongjian is between Taizong and his minister Wei Zheng. Wei Zheng repeatedly confronted the emperor with uncomfortable truths. He criticized Taizong’s lavish spending, his military adventures, and his personal conduct. Taizong, remarkably, listened. The Tongjian records dozens of these exchanges, building a portrait of a ruler who understood that his own judgment was fallible and that institutionalized disagreement was a tool of governance, not a threat to it.

Taizong also presided over a remarkable expansion of Tang influence. His armies defeated the Eastern Turks, extended Tang authority into Central Asia, and established a protectorate over the Western Regions. The Tang court received ambassadors from Byzantium, Persia, and dozens of other states. Chang’an, the Tang capital, was the largest and most cosmopolitan city in the world, with populations of Sogdians, Turks, Persians, and Arabs living alongside the Han Chinese majority. The Tongjian records this internationalism not as a cultural achievement but as a consequence of institutional strength. A state with strong institutions attracts talent and trade. When institutions weaken, the cosmopolitan facade crumbles.

When Wei Zheng died, Taizong is reported to have said: “With a mirror of bronze, you can adjust your attire. With a mirror of history, you can understand the rise and fall of states. With a mirror of people, you can know your own errors.” The Tongjian preserves this anecdote because it encapsulates Sima Guang’s own project. History is a mirror. The ruler who looks into it regularly is the ruler who governs well.

The An Lushan Rebellion: The Turning Point

The Tongjian‘s treatment of the Tang turns dramatically with the An Lushan Rebellion (755 to 763 CE). Before the rebellion, the Tang was the most powerful state in the world. After it, the Tang survived for another 144 years but never recovered its former strength. Sima Guang treats the rebellion as the central event of the Tang narrative, devoting extraordinary attention to its causes, its course, and its aftermath.

The causes, as the Tongjian reconstructs them, were institutional. Emperor Xuanzong, who had ruled brilliantly in his early decades, became complacent. He concentrated military power in the hands of regional commanders (jiedushi) who controlled both civil and military authority in their territories. He appointed An Lushan, a general of Sogdian origin, to command three of the most powerful military regions in the north. The Tongjian records the warning signs with precision: officials who warned against An Lushan’s growing power, the emperor’s refusal to listen, the court faction that protected An Lushan for its own purposes.

The rebellion killed millions. The Tang census recorded a population of roughly 53 million before the rebellion and about 17 million after. Even accounting for administrative disruption and lost territory, the demographic catastrophe was enormous. The Tongjian does not dwell on the numbers but on the decisions that led to them.

The Long Decline: 763 to 907 CE

The Tongjian covers the 144 years after the An Lushan Rebellion with unflagging attention. This is where the work’s method is most clearly demonstrated. Most histories would rush through a period of decline to get to the next dramatic event. Sima Guang does not. He tracks each crisis, each reform attempt, each minister who tried to reverse the slide and each emperor who accelerated it. The result is a detailed anatomy of institutional decay.

The key structural problem was the jiedushi system. Regional military commanders had gained autocratic power during the rebellion, and the Tang court could not reassert control. Some jiedushi became functionally independent, passing their commands to their sons without court approval. The court tried various strategies: dividing their territories, using eunuch armies to counterbalance them, forming alliances with some against others. None worked permanently. The Tongjian tracks the slow erosion of central authority with a patience that is itself a form of argument.

The eunuch problem compounded the military one. By the mid-9th century, eunuch factions in the palace had gained control of the imperial army and the ability to choose and depose emperors. The Tongjian records multiple coups where eunuchs killed emperors who tried to curb their power. The 830s and 840s saw a succession of puppet emperors, real power held by palace eunuchs, and regional commanders growing bolder. The most notable reform attempt came under Emperor Xianzong (r. 805 to 820 CE), who managed briefly to reassert control over several jidushi through a combination of military force and political maneuvering. His assassination by eunuchs in 820 CE ended the last serious effort at restoring central authority. The Tongjian treats his reign as a case study in how partial reforms can temporarily improve a situation while leaving the underlying structural problems intact, ensuring eventual relapse. For the lessons rulers drew from patterns like these, see Zizhi Tongjian’s Lessons for Rulers and Managers.

The Tongjian’s Verdict on the Tang

What does Sima Guang conclude from 289 years of Tang history? The Tongjian‘s conclusion is implicit but clear. The Tang rose because it inherited strong institutions from the Sui and was led by a ruler, Taizong, who respected those institutions and listened to criticism. It fell because subsequent rulers allowed institutional safeguards to erode, concentrated power in unchecked hands, and refused to confront structural problems until they became catastrophes. For comparison with the earlier model of history-writing, see Zizhi Tongjian vs the Shiji.

The Tang’s glory was real. Its poetry, its openness to foreign influence, its legal code, its administrative system were genuine achievements. But the Tongjian asks us to remember that glory is not a permanent condition. It is a product of specific institutional arrangements and specific human choices. When the choices change, the glory fades, and no amount of cultural achievement can substitute for the institutional discipline required to sustain a state. The Tang produced some of the finest poetry in any language, but the Tongjian reminds us that Du Fu wrote his greatest poems while fleeing a rebellion that killed millions. Cultural brilliance and political failure are not contradictions in the Tongjian’s view. They are simply different dimensions of the same history. The Tang is not a museum piece in the Tongjian. It is a mirror, and what it reflects is the permanent vulnerability of even the greatest states to the erosion of their own foundations.

FAQ

Why does the Tongjian devote so much space to the Tang?

The Tang provides Sima Guang with the richest material for his central argument about institutional strength and human judgment. It has a clear rise (Taizong), a turning point (An Lushan), and a long, documented decline. The density of sources for the Tang also makes detailed narrative possible. The Tongjian follows the sources; where they are rich, the narrative expands.

Does the Tongjian blame Emperor Xuanzong for the An Lushan Rebellion?

Not in the sense of a direct accusation. The Tongjian records the decisions that created the conditions for rebellion: concentrating military power in regional commanders, ignoring warnings, and allowing court factionalism to protect An Lushan. Sima Guang lets the pattern of decisions build the case. The implied judgment is that institutional failures, not personal malice, create the conditions for catastrophe.

What lessons does Sima Guang draw from the Tang decline?

The central lesson is that institutional safeguards matter more than individual brilliance. Taizong’s greatness lay not in his personal genius but in his willingness to respect institutions of criticism and constraint. Xuanzong’s failure was not a change in character but a gradual disregard for the safeguards that had made Tang governance effective.