Conquering Goguryeo had been an obsession with Emperor Taizong, and after his death in 649, his son Emperor Gaozong continued his ambition. After the Tang-Silla alliance conquered Baekje, Emperor Gaozong invaded Goguryeo in 661 and 662. One of Yeon Gaesomun's greatest victories came in 662, when his forces defeated Tang general Pang Xiaotai (龐孝泰) and his Lingnan army at the Sasu River (蛇水, probably Botong River). Pang Xiaotai and all his 13 sons were killed in combat. Famed Tang general Su Dingfang (蘇定方), who was instrumental in conquering Baekje, was unable to overcome Pyongyang's defenses and was forced to withdraw due to harsh snowstorms. With increasing domestic turmoil in China, Tang was once again forced to retreat.