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Rise to power
When scholar-official Kim Chi-yang burned the royal palace to the ground, threatening to kill the king and take over, King Mokjong called General Gang Jo to the capital city of Kaesong. General Gang Jo immediately executed Kim Chi-Yang and his supporters.
Then, the scholar officials, enemies of Gang Jo, spread rumors and lies that the general was planning to take over the government for himself. These rumors reached the King, and he planned to kill Gang Jo. Gang Jo heard of the conspiracy and doubts of the King, and ordered his army to attack and kill all of his enemies, including the King. After assassinating King Mokjong, General Gang placed King Hyeonjong of Goryeo on the throne.
War with the Liao and death
Soon after this, Emperor Shengzong of Liao attacked Goryeo during the fall of 1010 in an attempt to kill Gang Jo for killing the Goryeo King. The 400,000-man Liao army invaded Goryeo territory. The Liao first attacked and lost at the city of Uiju, whose castle lord was General Yang Gyu.
Next, the Liao finally headed to the city of Tong-Ju, which is where General Gang Jo and 30,000 Goryeo troops were waiting. General Gang Jo set up an ambush on a narrow pass that the Liao army was inevitably going to have to pass. There, he directly led his troops in a three-pronged attack when the Liao came. The Liao soldiers were forced to retreat and 10,000 died during this ambush. The enemy troops again attacked Tong-Ju city but faced a humiliating defeat with severe casualties.
The Liao commander led the remnants of his heavily-damaged army and led yet another attack on the city, with Gang Jo as his main target. The Liao were defeated a third time, and were forced to retreat once more. In one last-ditch effort, the Liao army came attacking once more, but this time, General Gang Jo did not directly orchestrate the attack and played Badok with one of his lieutenants instead, thinking that victory was a given. One of Gang Jo's men told his plan to the Liao. Liao soldiers pierced through the city's defenses and captured General Gang. General Gang refused to surrender to the Liao Emperor, who in turn executed Gang.
Place in history and comparison to Yeon Gaesommun
Gang Jo is seen as a hero. Though his rule was a time of war against the invading Liao, not many deaths were ordered by the general with the exception of King Mokjong and the conspiring scholar-officials. Gang Jo can be compared with his predecessor Yeon Gaesomun of Goguryeo, who had also killed a King of Goguryeo for conspiring against him.
Gang Jo, however, was not as brilliant as Yeon Gaesommun, as he was not able to keep control for very long. Gang Jo did bring great victories to Goryeo over the Liao dynasty, but his death brought about another period of trouble for Goryeo, just as Yeon Gaesommun's death had done to Goguryeo. Gang Jo can be seen as a smaller-scale version of Yeon Gaesomun.