We know this character has a story from earlier in his life, we’re not inventing one, and watching this super popular, super talented, kid go through the experience of training and war that turned him into the killing machine that he became would certainly be full of drama.
While prequels often seem like a last ditch effort to pull more success from a popular franchise when there’s nowhere left for the story to go, a prequel to Rambo doesn’t necessarily sound like the craziest idea.