 Hence, while Yeon Gaesomun was alive, Tang could not defeat Goguryeo.In 666 (though dates vary from 664–666), Yeon Gaesomun died of a natural cause and a civil war ensued among his three sons.His eldest son and immediate successor, Yeon Namsaeng, defected to Tang and provided the secrets and weaknesses of Goguryeo to Emperor Gaozong, and played a critical role in the next invasion and downfall of Goguryeo.