Meanwhile matters in Păk-je were becoming complicated again. A man named Pok Sin revolted against the government, proclaimed Pu-yŭ P‘ung, the son of a former king, monarch of the realm and planned a reëstablishment of the kingdom. This was pleasing to many of the people. So popular was the movement that the Emperor feared it would be successful. He therefore sent a summons to Sil-la to send troops and put it down. Operations began at once. Gen. Yu In-gwe besieged Ung-jin the stronghold of the pretender and chased him out, but a remnant of his forces intrenched themselves and made a good fight. They were however routed by the combined Sil-la and Chinese forces. But in spite of this defeat the cause was so popular that the country was honeycombed with bands of its sympathisers who gained many lesser victories over the government troops and their Sil-la allies. The Sil-la general, Kim Yu-sin, was very active, passing rapidly from one part of the country to another, now driving back to the mountains some band of Păk-je rebels and now holding in check some marauding band from Ko-gu-ryŭ. He was always found where he was most needed and was never at a loss for expedients. It is said that at this time rice was so plentiful in Sil-la that it took thirty bags of it to buy a single bolt of grass cloth. 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.