The aim of this research was to identify the reasons why North Korea and South Korea agreed to unite under a united Korean flag at the 2018 Asian Games. With the background of the dark history of these two countries, the unification of South Korea and North Korea is very significant considering These two countries have made peace efforts several times but ended in failure due to various factors, even in 2017 the tension between these two countries had been heated up due to a ballistic missile test conducted by North Korea which was considered to threaten the stability of the South Korean country. The researcher will try to identify why the two countries agreed to appear and compete in one delegation or one flag namely the United Korea flag, the author will identify this issue by using Rational Choice Theory by Graham T. Allison and Concept Soft Power Diplomacy by Joseph Nye Jr.