With the Korean War in full flight, things were not looking good for South Korea. The North Korean army had pushed south, all the way to the Nakdong River (낙동강) outside of Busan. The South Korean government began to ponder their options. They looked across the sea to Taiwan and made plans to become a government-in-exile should they need to flee Busan, and their plan was to use Jeju Island as their location.

However, the Syngman Rhee (이승만) administration remembered well the violence and fighting that occurred in Jeju following an uprising by Jeju citizens in 1948. The government was concerned that there were still communist elements in Jeju, and made plans to safeguard the island should they need to go there.

The government sent a secret message to the police on the island. In the order, the police were instructed to utilize the black lists they created earlier when the 4.3 Incident was at its peak in 1948/49 and preemptively arrest everyone on the list. The police went out and began arresting people. Upon being detained, people were given a rank of A, B, C, or D depending on their level of 'dangerousness'. Eventually the lower ranks of A and B were released. However people designated with the ranks of C and D were sentenced to death.

The idea behind this arrest and execution was something referred to as 'Preemptive Detention' (예비검속). The thought process is that people who were (allegedly) involved with 'negative' groups or actions in the past were likely to repeat them again in the future. So, if someone had had a run in with the police in the past or were associated with left wing groups in the past, they would likely cause problems with the police and government or support pro-leftist causes in the future. So, the police could arrest them in advance and 'prevent' them from causing problems later.

The executions took place over two periods. The first people were executed sometime around July 16-20, 1950 and the rest were executed on August 20, 1950. In the end, 191 people from the Daejeong area (대정읍) were executed. 63 people from the Hallim area (한림읍) were also killed.

One notable point about the event is that there were witnesses to the killing. Those who saw where the people were executed informed the families of those were killed. Some 400-500 people went to Seodal Oreum to collect their loved ones remains, but they were prevented from doing so by soldiers from the Army Counterintelligence Corps.

It wasn't until March 30, 1956 that the families of those from Hallim were able to collect their deceased relatives' remains. The families of those from Daejeong were able to do the same a month or so later. However, by this time the bodies had degraded to the point of being unrecognizable and DNA testing was not possible. As such, both groups collected the remains they could find (Hallim - 61 remains, Daejeong - 132 remains) and made two mass grave memorial sites.

In Hallim, the site is called 'Manbengdi Moseulpo Joint Burial Site' (만뱅디모슬포공동장지) but is simply referred to 'Manbengdi' in short. In Daejeong, the site is called 'Baekjoilson' (백조일손지) and the Chinese character meaning is literally 'One hundred ancestors one descendant'. It refers to the idea that when the remains were recovered they could not tell them apart. So from the one hundred ancestors (the killed family members) the surviving members became one descendant (one family).

References

Information Signs at Seodal Oreum/Alddeureu Airfield

Jeju 4.3 Archives - 만뱅듸공동장지 (readable here in Korean)

Jeju 4.3 Archives - 섯알오름 (readable here in Korean)

Jeju Dark Tours - Seodal Oreum (readable here in English)