In 612, the Sui Emperor Yangdi invaded Goguryeo with 1,133,800 men. However, Goguryeo continually resisted their enemy. Emperor Yangdi dispatched 305,000 men to Pyongyang, capital of Goguryeo. However, at this time, Goguryeo General 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) caused massive Sui casualties