It was written about Korea because the Vietnam War was still ongoing at the time so TV avoided overtly taking a side...yet, MASH was pretty close to doing just that. The historical context is really that movies critical of Vietnam didn't happen until 1979 or so with "Apocalypse Now." MASH predates that (particularly the movie with Elliot Gould and Donald Sutherland) by a longshot. The only other "Vietnam" movie of the era was The Green Berets with John Wayne in 1968, and that was a love letter to the war.