In 383 CE, one of the largest armies ever assembled in ancient China marched south toward the Yangtze River. The man leading it, Fu Jian, ruled over the Former Qin empire — the most powerful state China had seen in centuries. He commanded somewhere between 600,000 and 900,000 soldiers, depending on which source you believe, along with cavalry, supply trains, and all the apparatus of imperial might. His goal was simple: to conquer the remaining southern kingdoms and unify China under his rule for the first time in over a century.

Against him stood the Eastern Jin dynasty, a relatively small state clinging to the Yangtze River valley. Their army numbered perhaps 80,000 to 100,000 soldiers. They were outmatched in every measurable way — in numbers, in resources, in territory. Most strategists of the day gave them virtually no chance.

The Battle of Fei River — Féi Shuǐ Zhī Zhàn in Chinese — is the story of how the small side won anyway, and why that victory mattered more than almost any other battle you probably haven’t heard of.