The Siege of Kusong which occurred in 1231 was a decisive Goryeo victory against the Mongol Empire. After the Mongol army crossed the Yalu river, it quickly captured almost all of Goryeo's border defenses. The Mongol army however ran into stiff resistance both at Anju and the city of Kusong. According to an old Mongol general who, toward the end of the siege said: "...I have never seen [a city] undergo an attack like this which did not, in the end, submit."