Twenty years after the last film in the series, John Rambo has retreated to northern Thailand, where he’s running a longboat on the Salween River. On the nearby Thai-Burma (Myanmar) border, the world’s longest-running civil war, the Burmese-Karen conflict, rages into its 60th year. But Rambo, who lives a solitary, simple life in the mountains and jungles fishing and catching poisonous snakes to sell, has long given up fighting, even as medics, mercenaries, rebels and peace workers pass by on their way to the war- torn region. That all changes when a group of human rights missionaries search out the “American river guide” John Rambo…