In the television series, the 4077th Mobile Army Surgical Hospital is stationed near Uijeongbu, South Korea.
Uijeongbu is a real city north of Seoul, close to the military areas and routes leading toward the Korean front. The series uses the 4077th as a fictional unit, but places it within the real geography of the Korean War. Its location explains why wounded soldiers arrive constantly and why the surgeons can be sent to forward positions or temporary facilities.
M*A*S*H premiered on CBS in 1972 and ran for 11 seasons, from the Korean War setting of the early 1950s. The show deliberately used a fictional medical unit to tell stories about combat medicine, military bureaucracy, friendship, and the psychological strain of prolonged war.
Uijeongbu should not be confused with Daegu, Incheon, or Busan. Those are also major South Korean cities, but they are not the standard location identified for the television 4077th. The show's fictional camp is generally described as being near Uijeongbu, rather than inside the city itself.