Explanation: To fight with a river at one's back, leaving no avenue of retreat. It describes a situation where all avenues of withdrawal are cut off, forcing a fight to the death.
This idiom originates from "The Biography of Marquis of Huaiyin" in the *Records of the Grand Historian (Shiji)*: "Han Xin then dispatched ten thousand men as vanguard. They advanced and arrayed themselves with their backs to the river. The entire army fought with desperate courage, making defeat impossible."
Han Xin, from Huaiyin (present-day southwest of Qingjiang, Jiangsu), was a prominent general under Liu Bang, the King of Han. In order to defeat Xiang Yu and seize control of the empire, Han Xin devised a strategy for Liu Bang: first capturing the Guanzhong region, then crossing the Yellow River eastward to defeat and capture Wei Wangbao, a king who had betrayed Liu Bang and pledged allegiance to Xiang Yu. Following this, he advanced eastward to attack King Zhao Xie.
Han Xin's army needed to pass through a very narrow mountain pass known as Jingxing Pass. Li Zuoche, a strategist serving under King Zhao, proposed blocking the Jingxing Pass while simultaneously sending troops along a secret path to sever the Han army's supply lines and baggage trains. He argued that without reinforcements, Han Xin's expeditionary force would inevitably be forced into retreat. However, the Zhao general Chen Yu ignored this advice. Relying on his numerical superiority, he insisted on engaging the Han army in direct, head-on combat.
Upon learning of this, Han Xin was overjoyed. He ordered his troops to set up camp thirty *li* from Jingxing Pass. At midnight, he gave his soldiers a light snack, telling them they would eat a full meal after victory. Subsequently, he dispatched two thousand light cavalry to advance stealthily along a hidden path. Their orders were to rush into the Zhao army's camp the moment it was vacated, replacing all Zhao banners with Han flags. He also sent ten thousand troops to deliberately form a battle line with their backs to the river, intending to lure the Zhao forces into battle.
At dawn, Han Xin led his army in an attack, and fierce fighting erupted between the two sides. Soon, the Han troops feigned defeat and retreated back to their position by the river. The entire Zhao army left their camp to pursue. At this moment, Han Xin ordered his main force to launch a counterattack. The soldiers arrayed with their backs to the river, having no route of escape, turned fiercely upon the enemy. Unable to achieve victory, the Zhao army began to retreat toward their camp, only to find it suddenly filled with Han banners. Panic-stricken, they scattered and fled in all directions. The Han army pursued them in victory, winning a great triumph.
During the victory celebration, the generals asked Han Xin: "The Art of War says that an army should have mountains at its back and face marshes or water. Today, you ordered us to array our forces with our backs to the water and said we would eat our fill only after defeating the Zhao army. We didn't believe it at the time, yet we actually won. What strategy was this?"
Han Xin laughed and replied: "This is also mentioned in the Art of War; you simply didn't notice it. Doesn't the Art of War say, 'Plunge them into desperate straits, and they will survive; place them in mortal danger, and they will live'? If there were a path of retreat, the soldiers would all have fled. How then could I have made them fight with all their might?"
This story gave rise to the idiom "Back Against the River," which is primarily used in military contexts but can also be applied metaphorically to any decisive action or "fight to the finish."