I am a Vietnam Veteran, two tours. Back in the world meant leaving Nam and returning to civilization, home ("State Side") or elsewhere. "In Country" meant being there, in Nam . . . You were either "In Country", or "Back in the World". Not too complicated . . . . .
There was an exhibit of photographic portraits of Vietnam veterans in Wisconsin called "Back In the World," also...
Part of that phrase is that these brave men and women risked their lives for their country, and then came home to be ignored or even abused. One dear friend who is permanently physically disabled from Vietnam, also has severe PTSD, tells about coming home and one famous newscaster saying "The real Vietnam heroes were those who refused to fight and fled from the draft into Canada." It made him feel so desolate.
Here is an excerpt from that exhibit BACK IN THE WORLD, and I will leave a link also so that you can see some of their faces...which tell so much about their tragedy.
"Haunting portraits and emotional recollections set the tone and story of Back in the World, a traveling portrait exhibit that goes behind enemy lines to uncover the Vietnam War through the eyes of soldiers. The portraits’ intense gazes and solemn expressions exude the gravity of the display and bring visitors face-to-face with the men and women who experienced the horrors of war and the unwelcoming return to civilian life."