Got my interest up as I watched the show as a kid.. Found this bit of trivia.

Jim Nabors said that it was always difficult for him to watch the opening of the show because many of the men that he is seen marching with were killed in Vietnam
In an interesting turnaround, Frank Sutton, the actor whose best known role was playing a Marine sergeant, could not pass the Marine Corps. physical during WWII and ended up serving in the Army. While serving in the U.S. Army, Sutton distinguished himself by taking part in 14 assault landings, including Leyte, Luzon, Bataan and Corregidor.
Link this trivia

Carter's decorations are: Bronze Star (2), Purple Heart, Navy Presidential Unit Citation (2), Marine Corps Good Conduct Medal (3), World War Two Victory Medal, National Defense Service Medal, Korean Service Medal (4 Campaigns), UN Service Medal and a Korean Presidential Unit Citation. In addition, Carter wears Expert Rifleman and Expert Pistol marksmanship badges.