In 1231, Ögedei Khan ordered the invasion of Korea. The experienced Mongol army was placed under the command of General Saritai. The Mongol army crossed the Yalu river and quickly secured the surrender of the border town of Uiju. The Mongols were joined by Hong Bok-won, a traitor Goryeo general. Choe Woo mobilized as many soldiers as possible into an army consisting largely of infantry, where it fought the Mongols at both Anju and Kuju (modern-day Kusong). The Mongols took Anju; however, they were forced to retreat after the Siege of Kuju. Elements of the Mongol army reached as far as Chungju in the central Korean peninsula; however, their advance was halted by a slave army led by Ji Gwang-su where his army fought to the death. Realizing that with the fall of the capital Goryeo was unable to resist the Mongol invaders, Goryeo sued for peace. There were six major campaigns: 1231, 1232, 1235, 1238, 1247, 1253; between 1253 and 1258, the Mongols under Möngke Khan's general Jalairtai Qorchi launched four devastating invasions against Korea at tremendous cost to civilian lives throughout the Korean peninsula.