It was at the close of this successful campaign in Shansi, that Taitsong, in the year 1635, assumed, for the first time among any of the Manchu rulers, the style of Emperor of China. Events had long been moving in this direction, but an accident is said to have determined Taitsong to take this final measure. The jade seal of the old Mongol rulers was suddenly discovered, and placed in the hands of Taitsong. When the Mongols heard of this, forty-nine of their chiefs hastened to tender their allegiance to Taitsong and the only condition made was that the King of Corea should be compelled to do so likewise. Taitsong, nothing loth, at once sent off letters to the Corean court announcing the adhesion of the Mongols, and calling upon the king of that state to recognize his supremacy. But the Corean ruler had got wind of the contents of these letters and declined to open them, thus hoping to get out of his difficulty without offending his old friends the Chinese. But Taitsong was not to be put off in this fashion. He sent an army to inflict chastisement on his neighbor, and its mission was successfully discharged. The king and his family were taken prisoners, although they had fled to the island of Gangwa for safety, and Corea became a Manchu possession. The last years of Taitsong's life were spent in conducting repeated expeditions into the provinces of Shansi and Pechihli, but the strength of the fortresses of Ningyuen and Shanhaikwan on the Great Wall effectually prevented his renewing his attempt on Pekin. These two places with the minor forts of Kingchow and Songshan formed a quadrilateral that effectually secured Pekin on its northern side, and being intrusted to the defense of Wou Sankwei, a general of great capacity, of whom much more will be heard, all Taitsong's ability and resources were taxed to overcome those obstacles to his progress south of the Great Wall. He succeeded after great loss, and at the end of several campaigns, in taking Kingchow and Songshan, but these were his last successes, for in the year 1643 he was seized with a fatal illness at Moukden, which terminated his career at the comparatively early age of fifty-two. Taitsong's premature death, due, in all probability, to the incompetence of his physicians, cut short a career that had not reached its prime, and retarded the conquest of China, for the supreme authority among the Manchus then passed from a skillful and experienced ruler into the hands of a child.