Because First Blood went on to create a franchise in which the character of John Rambo's muscles and guns got bigger, it's easy to forget that his very first adventure was not an adventure at all. When we hear the name Rambo, we picture the head-banded Lothario running through jungles with assault rifles or AK-47s shooting holes in any manner of ethnic groups. But First Blood, the first film to feature the character of John Rambo, was not such a film. It was actually a very political morality tale about the horrors of the Vietnam War and how it completely fucked up the minds of many soldiers, either on the battlefield or in the years to follow their arrival home.
And First Blood once had a very downbeat ending, one that I believe reflected the ending of the novel by David Morrell, upon which the film is based. And, had this original ending been the one used (the new ending having caused Kirk Douglas, the original Col. Trautman, to quit the shoot), we never would have had further adventures with John Rambo.