Yeon Gaesomun
In the winter of 642, King Yeongnyu was apprehensive about Yeon Gaesomun, one of the great nobles of Goguryeo, and plotted with other officials to kill him. However, Yeon Gaesomun caught news of the plot and killed Yeongnyu and 100 officials, initiating a coup d'état. He proceeded to enthrone Yeongnyu's nephew, Go Jang, as King Bojang while wielding de facto control of Goguryeo himself as the generalissimo (). Yeon Gaesomun took an increasingly provocative stance against Silla Korea and Tang China. Soon, Goguryeo formed an alliance with Baekje and invaded Silla, Daeya-song (modern Hapchon) and around 40 border fortresses were conquered by the Goguryeo-Baekje alliance. In 643, under pressure from the Goguryeo–Baekje alliance, Silla requested military aid from Tang. In 644, Tang began preparations for a major campaign against Goguryeo.