First Blood - The 1982 film based on the novel of the same name, starring Sly Stallone in his first outing as John Rambo. This is a great action-thriller, far grittier in tone than the movies that followed, and far more grounded too.
Rambo is a Vietnam vet trying to track down members of his old unit. As he tries to walk through a small mountain town he runs into local sheriff Brian Dennehy. At first the sheriff tries to "help" Rambo by giving him a lift to town limits, but when Rambo tries to return to the town, the sheriff arrests him and then all hell breaks loose.
I've not read the novel, but I understand there are some differences, notably that Rambo survives (in the book Trautman kills him). However, the underlying theme of Vietnam vets, once treated as war heroes, are now forgotten and unwanted by society, plus the effects of PTSD are strongly highlighted throughout the film, and Rambo cuts more of a troubled lonely figure trying to find a place for himself in the world, rather than the one-man army he portrays in later outings.
If you want to see a more vulnerable Rambo character, then First Blood won't disappoint. The cast is good, the acting solid, all ably directed by Ted Kotcheff.