An old war movie where 1 American and Japanese soldier are dropped off on an island to battle each other its not hell in the pacific?

You're likely thinking of the 1965 film "The Dirty Dozen". While not the central plot, the film does include a scene where two soldiers, an American and a Japanese, are dropped onto a deserted island to fight to the death. This was part of a training exercise for the "Dirty Dozen" squad, a group of condemned soldiers recruited for a dangerous mission.

While not the main focus of the film, this scene is memorable and aligns with your description. It's a good example of how war movies often explore the brutality and psychological toll of conflict.