for several months at Ansi Fortress due to the resistance of the celebrated commander Yang Manchun. Second, the elite marine force that he sent to take Pyongyang, Goguryeo's capital, was defeated by Yeon Gaesomun who, according to the Joseon Sanggosa, then immediately marched his legions to relieve Yang Manchun's forces at Ansi Fortress. Emperor Taizong, caught between Yang Manchun's army in the front and Yeon Gaesomun's counter-attacking forces closing in from behind, as well as suffering from the harsh winter and dangerously low food supplies, was forced to retreat homeward. Before setting off, Emperor Taizong left behind 100 bolts of silk cloth out of respect to Yang Manchun. The retreat was difficult and many of his soldiers died. Emperor Taizong's campaign against Goguryeo was unsuccessful. However, he succeeded in inflicting heavy casualties on Goguryeo. Upon returning home, Emperor Taizong founded the Minzhong Temple, the oldest temple in Beijing, to commemorate his soldiers who died in Goguryeo. He invaded Goguryeo again in 647 and 648, but was defeated both times, and thus was unable to accomplish his ambition of conquering Goguryeo in his lifetime. 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 () and his Lingnan army at the Sasu River (, probably Botong River). Pang Xiaotai and all his 13 sons were killed in combat. Famed Tang general Su Dingfang, 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. However, Goguryeo's population and economy were severely damaged due to the long years of continuous warfare. Yeon Gaesomun died in 666 of a natural cause, and Goguryeo was thrown into chaos and further weakened by a succession struggle among his sons and younger brother, with his eldest son defecting to Tang and his younger brother defecting to Silla. Tang mounted a fresh invasion in 667, aided by Silla and the defector Yeon Namsaeng, and was finally able to conquer Goguryeo in 668. Death The most likely date of Yeon Gaesomun's death is that recorded on the tomb stele of his eldest