The gate spawned on Jeju Island about four years before the events of the main storyline. Due to how weak the Korean S-Rank Hunters were at the time, they failed to close the gate and it eventually underwent a dungeon break, resulting in the deaths of thousands of Korean civilians and turning the island into a wasteland no longer fit for human habitation.
Over the next three years, the Korean government launched three search-and-destroy missions on the island to no success. When the third raid resulted in the death of Eunseok, one of their prized S-Rank Hunters, the Korean government formally abandoned their efforts to reclaim Jeju and decided to leave the island be in hopes that the ants would eventually perish on their own by starving to death when their food supply eventually ran out. Unfortunately, this move ultimately backfired, as the ants eventually evolved and gained the ability to fly long distances.
In response to this evolution, the Draw Sword Guild, Japan's strongest and largest guild, agreed to collaborate with the Koreans in liberating the island before all of the ants learned how to take flight. A simple but promising plan was worked out; the Japanese hunters would use themselves as bait to draw out the ants, while the Korean hunters would sneak into the unguarded nest and kill the queen.
Initially, the raid went well, with the Japanese easily slaughtering the ants and the Koreans killing the queen and her guards with minimal trouble. However, everything went horribly wrong when an immensely powerful ant called the Ant King ambushed both teams and easily slaughtered eight of the sixteen S-Ranks who were participating in the raid, including Goto Ryuji and Min Byung-Gyu. As a result, the raid nearly failed altogether and the entire party would have been slaughtered had it not been for Sung Jinwoo, who swiftly arrived on the island and killed the Ant King after a fierce fight.
With the boss of the dungeon finally defeated, the raid party retreated back home, with only Jinwoo staying behind to finish off the rest of the ants. After Jinwoo completed his work and returned back to Korea, the Korean Hunters Association sent in the Knights Guild to collect all the essence stones on Jeju and to make sure that there was no ants left alive.
The fact that the 4th Jeju Island Raid had only succeeded thanks to Sung Jinwoo made him a national hero in Korea. His fight with the Ant King was also broadcast all over the world for everyone to see, cementing his status as one of the strongest hunters in Asia, and caused a major geopolitical shift, with nations as powerful as America and China wanting to snatch Jinwoo for themselves.
Although the raid was a success, Byung-Gyu's death still took its toll and he was honored as a hero in a national funeral that was held for him in Seoul. Jinwoo was also blamed by a good handful of Korean netizens for Byung-Gyu's death, as he probably wouldn't have died had Jinwoo participated in the raid from the start. However, at the end of the day, significantly more Koreans had a positive perception of Jinwoo than a negative one and he gained a great deal of respect from Korean society as a result.
In Japan, however, the events of the 4th Jeju Island Raid had a catastrophic effect on the country. The deaths of seven of the Draw Sword Guild's S-Rank Hunters not only resulted in the downfall of the guild altogether, but also caused a power struggle with the other major guilds in Japan. Goto's death also meant that Japan had lost a great deal of their military power and as a result, the country was left wide open when a S-Rank Gate suddenly spawned in Tokyo several months later.