The Battle of Salsu was an enormous battle that occurred in the year 612 during the second Goguryeo–Sui War between the Korean kingdom of Goguryeo and Sui China. Goguryeo cavalry forces, pursuing the Sui army, attacked and defeated it at the Salsu River.
In AD 612 the Sui Emperor Yangdi invaded Goguryeo with over one million men. However, Goguryeo defiantly resisted its enemy. Emperor Yangdi dispatched 305,000 troops to Pyongyang, capital of Goguryeo.
Goguryeo Gen. Eulji Mundeok defended fortresses against the Sui army and navy for several months and destroyed the Sui troops while retreating into Goguryeo territory. An ambush at Salsu (Chongchon River) inflicted massive casualties on the Sui forces. When the Sui army had reached Salsu the water level was shallow, as Eulji Mundeok had already cut off the flow of water with a dam. When the Sui troops were halfway across the river, Eulji opened the dam and the onslaught of water drowned thousands of Sui soldiers. The Goguryeo cavalry then charged the remaining Sui forces.
With victory over the Sui dynasty at the Salsu, Goguryeo eventually won the war itself, while the Sui Dynasty, crippled by the enormous loss of manpower and resources as a result of its Korean campaigns, started to crumble from within and was finally brought down by internal strife, to be replaced soon thereafter by the Tang.