The Power Struggle in Wei: Prelude to a Failed Campaign

In the year 244 CE, two decades before Sima Zhao’s decisive conquest of Shu, the Wei general Cao Shuang launched a massive but ultimately disastrous invasion of the rival state. This campaign, often overshadowed by later events, marked a critical turning point in the Three Kingdoms period—one that exposed the fragility of Wei’s political order and set the stage for the Sima clan’s rise.

Cao Shuang, son of the renowned general Cao Zhen, had been appointed regent alongside Sima Yi following the death of Emperor Ming of Wei. The young Emperor Fang’s reign became a battleground for these two powerful figures. After sidelining Sima Yi through political maneuvering, Cao Shuang sought to cement his authority through military glory. Conquering Shu, he believed, would silence his critics and secure his dominance.

The Invasion Plan: Ambition Meets Geography

Cao Shuang’s strategy relied on surprise. He mobilized 100,000 troops in Chang’an, amassed supplies, and chose an unconventional route—the treacherous Tangluo Trail—to invade Hanzhong, Shu’s northern stronghold.

The Qin Mountains separating Wei and Shu offered several invasion routes, each with its own challenges:
– Ziwu Dao: Direct but heavily guarded.
– Baoxie Dao: Frequently used by Zhuge Liang during his northern campaigns.
– Tangluo Dao: The least traveled, featuring 420 li (approx. 210 km) of narrow paths, 84 winding passes, and three critical choke points.

Shu’s defenses in Hanzhong were stretched thin, with only 30,000 troops. Had Cao Shuang achieved surprise, victory might have been possible. But secrecy proved impossible with such a large force.

Shu’s Defense: A Masterclass in Tactical Resistance

Under the command of veteran general Wang Ping, Shu’s forces fortified the southern entrance of Tangluo Dao at Xingshi Mountain, a natural stronghold. Wei’s supply lines, stretched across the narrow trail, collapsed under logistical strain. Horses and transport animals perished, and starvation loomed.

Forced into retreat, Cao Shuang’s army suffered heavy losses from Shu’s ambushes. The campaign ended in humiliation, weakening Cao Shuang’s political standing and hastening his eventual downfall.

The Fall of Cao Shuang and Sima’s Ascendancy

Cao Shuang’s failure became a gift to Sima Yi. In 249 CE, Sima Yi staged a coup, executing Cao Shuang and his faction. The Sima clan then systematically dismantled Wei’s imperial authority:
– Sima Shi deposed Emperor Fang in 254 CE.
– Sima Zhao removed Emperor Cao Mao in 260 CE.

Resistance flared, particularly in Shouchun, a frontier region with loyalist generals. Three rebellions—led by Wang Ling, Guanqiu Jian, and Zhuge Dan—were crushed, consolidating Sima control.

The Broader Three Kingdoms Context

While Wei fractured internally, Shu and Wu faced their own crises:
– Shu: Emperor Liu Shan’s incompetence and宦官 Huang Hao’s corruption eroded governance. Jiang Wei’s costly northern campaigns drained resources without gains.
– Wu: After Sun Quan’s death, regents like Zhuge Ke launched failed invasions, triggering internal strife.

Shu’s defensive strategy in Hanzhong deteriorated. Once a network of frontier outposts, it collapsed into a desperate “turtle shell” defense—garrisoning troops in Han and Yue cities while abandoning key passes. This fatal weakness would later be exploited by Wei.

The Overlooked Genius: Deng Ai’s Strategic Vision

Amid these upheavals, one figure stands underappreciated: Deng Ai, a stuttering orphan turned military savant. His insights shaped Wei’s eventual victory.

### The Economics of Conquest
Deng Ai’s 243 CE treatise, Ji He Lun, argued that unification required overwhelming economic superiority. He calculated that defeating Shu and Wu demanded:
– 10万 troops sustained for 5 years.
– 3 million dan (approx. 180,000 tons) of grain reserves.

### The Shouchun Miracle
Deng Ai identified Shouchun—a war-ravaged but fertile region—as Wei’s future breadbasket. By mobilizing 50,000 soldiers to repair irrigation systems, he boosted yields threefold, creating an annual surplus of 500,000 dan. Within six years, Wei stockpiled enough grain to fund Sima Zhao’s conquests.

### Predicting the Unraveling of Wu
When Wu’s Zhuge Ke retreated after a failed 253 CE siege of Hefei, Deng Ai alone predicted his imminent downfall:
“He prioritized external wars over consolidating power. Now, having wasted lives for no gain, his own people will destroy him.”
Weeks later, Zhuge Ke was assassinated in a coup.

Legacy: The Invisible Foundations of Unification

Deng Ai’s logistical brilliance and Cao Shuang’s miscalculations reveal a deeper truth: the Three Kingdoms’ fate was decided not just on battlefields, but in granaries and canals. By the 260s, Wei—now under the Sima clan—held an insurmountable material advantage.

When Deng Ai later circumvented Jiang Wei’s defenses to conquer Shu in 263 CE, it was the culmination of decades of preparation. His legacy, like Cao Shuang’s folly, reminds us that history often turns on the unseen—the patience of planners and the hubris of would-be conquerors.

The Tangluo Trail remains a silent witness: a path where ambition faltered, and where the seeds of Jin’s unification were sown.