The Emperor listened to and profitted by this advice, for during the events to be related his soldiers never suffered from over-confidence, but in their advances made sure of every step as they went along.
Active operations began by the sending of an army of 40,000 men in 501 boats to the harbor of Nă-ju where they were joined by land forces to the number of 60,000, besides large contingents from the wild tribes of the north. Large numbers of ladders and other engines of war had been constructed and were ready for use. Before crossing the Liao River the Emperor made proclamation far and wide saying “Hap So-mun has killed our vassal, King of Ko-gu-ryŭ, and we go to inquire into the matter. Let none of the prefects along the way waste their revenues in doing us useless honors. Let Sil-la, Păk-je and Kŭ-ran help us in this righteous war.”
Crossing the Liao without resistance the Chinese forces marched toward the fortress of Kön-an which soon fell into their hands. Some thousands of heads fell here to show the rest of Ko-gu-ryŭ what they might expect in case of contumacy. Then Ham-mo Fortress fell an easy victim. Not so the renowned fortress of Liao-tung. As the Emperor approached the place he found his way obstructed by a morass 200 _li_ in length. He built a road through it and then when all his army had passed he destroyed the road behind him as Pizarro burnt his ships behind him when he landed on the shores of America to show his army that there was to be no retreat. Approaching the town he laid siege to it and after a hard fight, during which the Chinese soldiers lifted a man on the end of a long piece of timber until he could reach and set fire to the defences that surmounted the wall, an entrance was finally effected and the town taken. In this battle the Chinese were materially aided by armor which Păk-je had sent as a gift to the Chinese Emperor.
The Chinese were destined to find still greater difficulty in storming An-si Fortress which was to Ko-gu-ryŭ what Metz is to Germany. It was in command of the two generals, Ko Yŭn-su and Ko Hye-jin who had called to their aid 100,000 warriors of the Mal-gal tribes. At first the Emperor tried a ruse to draw the garrison out where he could give them battle. The wise heads among the Ko-gu-ryŭ garrison strongly opposed the sortie saying that it were better to await an opportunity to cut off the Chinese from their base of supplies, and so entrap them; but they were outvoted and the greater part of the Ko-gu-ryŭ and allied forces marched out to engage the enemy in the open field. The Emperor ascended an eminence where he could obtain a view of the enemy and he beheld the camp of the Mal-gal allies stretching out forty _li_, twelve miles. He determined to exercise the utmost caution. One of his generals, Wang Do-jong begged to be allowed to march on P‘yŭng-yang, which he deemed must be nearly bare of defenses, and so bring the war to a speedy close; but the Emperor, like Hannibal when begged by his generals to march straight into Rome, made the mistake of over-caution and so missed his great opportunity. To the Emperor this sounded too much like a similar attempt that had once cost China 300,000 men.