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 raids to no success. When the third raid resulted in the death of Eun-Suk, the Korean government formally abandoned their efforts to reclaim Jeju Island 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 eventually ran out. Unfortunately, this idea eventually backfired with the ants evolving to the point that they could fly long distances.
In response to this evolution, the Draw Sword Guild, Japan's largest and most powerful guild, agreed to collaborate with the Koreans in liberating the island before all of the ants learned how to fly. 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 until Sung Jin-Woo arrived on the island and easily killed the Ant King in a fierce 1v1 fight.
Following the Ant King's death, Jin-Woo stayed behind to finish off the surviving ants who were still on the island. After successfully doing so, he extracted the Ant King's shadow to add to his army, named him Beru when the shadow demonstrated the ability to speak, and sent him out to slaughter the ants who had managed to flee the island during the raid.
The fact that the 4th Jeju Island Raid had only succeeded thanks to Jin-Woo made him a national hero in Korea and one of the strongest hunters in Asia, only surpassed by Liu Zhigang and Siddharth Bachchan at that point. His fight with the Ant King was also broadcast all over the world for everyone to see and caused a major geopolitical shift, with nations as powerful as America and China wanting to snatch Jin-Woo for themselves.
Although the raid was a success, Byung-Gyu's death still took its toll and he was honored in a national funeral that was held for him in Seoul. Jin-Woo was also blamed by a good handful of Korean netizens for Byung-Gyu's death, as he probably wouldn't have died had Jin-Woo participated in the raid from the start. However, at the end of the day, significantly more Koreans had a positive perception of Jin-Woo 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 Draw Sword Guild altogether, but also caused a power struggle with the other 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.