My main focus is Early Medieval China, and when you talk about grave miscalculations, the Former Qin-Eastern Jin War immediately comes to mind. Not only is it possibly the most decisive military confrontation in all of Chinese history, but the historiography surrounding the climactic final confrontation at Fei River is fascinating.

Warning: rant ahead. Part 1 is the cool fighting stuff, Part 2 is the historiography crap that nobody cares about.

Part 1: The Story (aka what I assume you came for)

Let’s set the scene. It’s the 4th century CE, and China is lost. In the north, nomadic tribes have invaded or rebelled their way to sovereignty, splitting the Central Chinese Plain into a multitude of ferocious yet poor states. In the south, Sima Rui has led the remnants of the Jin Dynasty to Jiankang, where they lick their wounds amidst reluctant Southern allies, still hoping for a return home.

Into this world enters Fu Jian.

He is born in 337 a member of the Former Qin royal family, belonging to the Di confederation. The Di, historically fiercely disunited and without the nomadic advantages of the Xianbei or Xiongnu, are not looked upon terribly optimistically by foreign observers. Yet in 357, Fu Jian changes all this when his successful coup earns him the throne. Tremendously competent and unfailingly ambitious, Fu Jian rapidly doubles his territory by conquering Murong Yan in 370. Jin outposts in Sichuan are hit in 373, the kingdoms of Former Liang and Tuoba Dai in 376, and by 381 there is not a part of Northern China free of his rule. And as soon as he is done with what no one else has managed in only 24 years, he prepares to invade the South. This man would have gone down as China’s Napoleon…

Had he won.

Former Qin begins southwards by sending his son Fu Pi to take the important fortress of Xiangyang. He does, in 379. The foundations of Southern defense are dozens of forts along the Huai River, rather symbolically just about between the Yellow and Yangtze Rivers. Fu Jian, ever the strategist, realizes this, and moves to take as many forts as possible before committing his forces fully into the humid river valleys of Eastern Jin. Jin itself knows what Qin is trying, and pushes its forces back above the Huai before they threaten the Southern capital at Jiankang. With the Jin counterattack culminating in a failure to retake Xiangyang, Fu Jian immediately moves to launch his offensive.

Fu Jian gathers men from all over his country to Chang’an. According to the Chronicle of Fu Jian from the Jin Shu, 270,000 cavalrymen and 600,000 infantry arrive to fight his war, apparently a tenth of all registered adult males in the North. Former Qin concentrates numbers towards the city of Shouchun, supplementing some 250,000 troops already there and 50,000 intended to prevent further Jin counterattack at Luo Creek.

Here we meet Fu Jian’s legendary nemesis: Xie An. A vastly talented politician who dominated the court during this era, he is often depicted as a charismatic and cultured philosopher who effortlessly dispels his enemies with chants. He leads a force of only 70,000 along with some of his most trustworthy relatives and allies to confront the onslaught of the North.

Xie An begins his defense with an offense; spotting the Qin camp, he immediately launches a nighttime attack, surprising an enemy commander, killing him and 5,000 enemy troops.

Later, Xie An finally marches to a large river in front of Shouchun, where he is greeted to the sight of the city being held by the massive force Fu Jian had gathered from Chang’an. Knowing that Fu Jian is the superior tactician and that his forces are hopelessly unmatched on the field, he devises a plan to win the city without any conflict. Xie An sends a messenger to ask that his forces be allowed to ford the river to fight the Qin army, so that a lengthy stalemate can be avoided. Fu Jian, seeing the message, agrees. After all, in classical Chinese military thought, sieges are thought to be the lowest form of warfare, and as an experienced attacker, Fu Jian knows that he needs to seize the momentum if he wants to beat an enemy with a far larger agricultural base. So his generals sound the horn to retreat from the city, and he creates a plan to attack Jin before they fully cross the river.

But when his younger brother Fu Rong calls for a withdrawal, his soldiers, largely northerners unfamiliar with the capabilities of the south, instantly panic at the sound of an army that has previously killed 5,000 of their number at the gates. So the calm retreat Fu Jian had envisioned quickly becomes a mess, and by the time he gets his men back in some semblance of order… the Jin army has arrived.

With the Qin army still not in fighting shape, and now outside of the mighty Shouchun city walls, Xie An easily tears through the disorganized enemy. It is a decisive and utter rout, as northern soldiers continue fleeing from southern soldiers slaughtering their way into their midst.

And worst of all, it breaks Fu Jian’s carefully crafted image of invulnerability.

Fu Jian’s strategy, all along, to maximize his chances of success, has been to incorporate all those who he defeats or who surrender to him. His enemies, from the Murong family to the Tabgatch clan, have all been integrated into his command structure to encourage them not to revolt against the man who so handedly defeated them. Even worse, he’s allowed many local warlords to only vassalize themselves to him, without confiscating their armies and land, meaning in the north, tensions immediately explode. After all, with such a large army, Fu Jian could not even defeat the Southern savages. So why should he be invincible now that his large army is gone?

The Murong are the first to rebel, quickly restoring their home kingdom of Yan and overtaking Former Qin’s original territories. In the west, Fu Jian’s Di commanders take Liang for themselves, and Xianbei and Qiang leaders claiming to serve Former Qin war for the right of being called its legitimate successor. The winds of time once again show that in the grinding thrust of the dynastic cycle, one wrong mistake can tear all your hard work asunder in an instant.

But this is not the end of Fu Jian and his methods. In the north, the Tabgatch Xianbei clan rebels along with every other kingdom he has conquered, and they do something others don’t: they learn from his successes and failures. While the western states fight over the right to look like Qin in name, the state of Dai, now renamed Wei, does with the Xianbei tribesmen what Former Qin did with the Di, dissolving old tribes and building new heterogeneous ones to only serve the state. By their fifth century, these have earned them the north. Knowing the failures of Fu Jian, they seek to attack the south only in increments. This earns them a century of unity, more than can be said for any other Northern state.