Hong Ta-gu was arrested soon after his father's death in 1260 for trying to provoke conflict between Goryeo and the Mongols by saying that the Goryeo king had lied about his intentions to move the capital back to
Kaesong. In 1261, Kublai summoned Hong Ta-gu and told him that his father had been unjustly punished. Hong Ta-gu was made
chonggwan of the Goryeo warriors. This was probably due to political considerations. There were a few tens of thousands of Goryeo warriors in
Liaoyang and
Shenyang, and Kublai demanded their loyalty under threat of punishment for their families. In 1263, Hong Ta-gu reported that Wang Jun, the Goryeo royal in command of Goryeo families in Shenyang, boasted that his position was no less than that of the heir apparent. Wang Jun's command was stripped and given to Hong. According to Goryeo sources, Hong "hated his motherland". In 1269, when Im Yon attempted to remove Wonjong from power, Hong received orders to invade Goryeo with 3,300 men. This campaign opened up Goryeo to direct Yuan management of internal affairs and Hong and his Goryeo warriors became one of the primary instruments of such policy for about ten years. In 1270, he was part of the negotiations behind marrying the Goryeo king to a Mongol princess. In 1271, he participated in the suppression of the
Sambyeolcho Rebellion with particular zeal. Wang Jun had instructed his sons, who participated in the campaign, to save Wang On, a Goryeo prince who the rebels had named their king. Hong was the first to land on
Jindo Island to attack the rebel base and he personally killed Wang On. He also refused to bow to the Goryeo king when he visited court that year. In 1273, Hong carried out the mass slaughter of rebel prisoners when Yuan and Goryeo troops took
Tamna, the last stronghold of the Sambyeolcho. Hong participated in the
Mongol invasions of Japan. In April 1274, the Yuan instructed Holdon and Hong Ta-gu to mobilize 15,000 men for the invasion of Japan. Hong was put in charge of the construction of large transports and was harsh and cruel in exploiting the Goryeo people for the project. Hong's people looted Goryeo dwellings while preparing for the invasion. In 1277, Goryeo officials reported that food prices rose due to Hong's looting. In 1278, Hong requisitioned rice and beans from western Goryeo. In 1279, a Goryeo envoy reported that the people were distracted by Hong's atrocities. During the actual invasion, one of the three commanding Yuan generals, Liu Fuxiang (Yu-Puk Hyong), was shot in the face by retreating samurai and seriously injured. Liu convened with the other generals Holdon and Hong back on his ship. Holdon wanted to keep advancing through the night before more Japanese reinforcements arrived, but Hong was worried that their troops were too exhausted and needed rest. There was also fear of being ambushed in the night. Liu agreed with Hong and recalled the Yuan forces back to their ships. Due to a storm, many of the Yuan ships were beached and destroyed. Of the 30,000 strong invasion force, 13,500 did not return. Hong also led the Eastern Route army along with Kim Pang-gyŏng during the second invasion. They disobeyed orders to wait for the Southern Route army and attacked the Japanese mainland by themselves. They failed. After Hong's failure in Japan, his standing declined. He attacked a popular Goryeo general, Kim Pang-gyŏng, who had fought the Mongols and afterwards sought establishment of friendly ties with the Yuan court. Kim also led Goryeo forces in the Sambyeolcho Rebellion and invasion of Japan, which naturally threatened Hong's position. Hong and Kim ran afoul of each other due to Hong's harsh tendencies during the preparation for the invasion of Japan. When a scandal connected Kim to an alleged plot to have the king murdered, Hong arrested Kim, tortured him, and requested permission from the Yuan court for another campaign against Goryeo.
Chungnyeol of Goryeo and Hong were summoned to
Khanbaliq, where a discussion took place and Kublai sided with the king. The Mongol troops were called off, the king returned to Goryeo, and Hong's supporters were exiled. After 1281, there are no Goryeo sources about Hong Ta-gu. ==Legacy==