That same autumn the Chinese engaged the Ko-gu-ryŭ forces at the Yalu River and gained a decided victory. Then the fortress at Ma-eup San fell into their hands. This cleared the road to P‘yŭng-yang, and the Chinese boldly advanced and laid siege to that ancient stronghold. At the same time the Emperor ordered Sil-la to send troops to coöperate with the imperial army. She obeyed, but with great trepidation, for the fame of Ko-gu-ryŭ’s arms made this seem a matter of life and death. She was obliged to comply, however, or lose all the vantage ground she had gained in the Emperor’s favor. There were still some Ko-gu-ryŭ forces in the north and they were attempting to check the advance of a large body of Chinese reinforcements. It was late in the autumn and the Yalu was frozen. Taking advantage of this the Chinese crossed in the night and falling suddenly upon the unsuspecting army of Ko-gu-ryŭ inflicted a crushing defeat. It is said that 30,000 Ko-gu-ryŭ soldiers were killed in this engagement. The speedy downfall of Ko-gu-ryŭ seemed now inevitable, but a sudden timidity seized the Emperor, who feared perhaps to let his army winter on Korean soil. So he sent orders for an immediate retreat back to Chinese territory. The generals before P‘yŭng-yang were deeply chagrined and indeed found it impossible on account of lack of provisions to obey the command at once. Soon the Sil-la army arrived before P‘yŭng-yang with full supply of provisions. These the Chinese took and the greater part of them reluctantly broke camp and marched back to China, leaving Sil-la in a frame of mind better imagined than described.
While Ko-gu-ryŭ was staggering under the terrible reverses inflicted by the Chinese, events of interest were taking place in the south. The kingdom of T‘am-na on the island of Quelpart had always been a dependency of Păk-je, but now found it necessary to transfer her allegiance to Sil-la. The king of T‘am-na at that time was To-dong Eum-yul.
The mischief-maker, Pok-sin, was again in the field. Now that he was relieved of pressure he came back to the charge and took Ung-jin from the Chinese. At the earnest request of the governor the Emperor sent Gen. Son In-sa with a small army to aid in putting down this dangerous malcontent. Pok-sin was obliged to retire to Chin-hyŭn where he fortified himself strongly. Success seems to have turned his head for he began to carry himself so proudly that his followers arose and put him to death and then sent a messenger to Ko-gu-ryŭ and to Japan asking aid against the Chinese. The latter responded by sending a considerable force to the shores of Păk-je to coöperate with this hardy band of men who were honestly fighting for the independence of their country.