The Gisa Hwanguk (Korean: 기사환국) occurred when the Westerners fell out of power after opposing the naming of a crown prince by Sukjong of Joseon. Prominent Westerner Song Si-yeol and others were executed. Shortly after, the Westerners split into Noron (Song's supporters within the Westerners) and Soron (Song's opponents within the Westerners).
References
Gisa Hwanguk Wikipedia(Text) CC BY-SA