Put yourself in the place of an American GI in a "civilized" enemy's hands. You would expect them to employ some things like sensory and sleep deprivation, solitary confinement, unpleasant conditions, a monotonous environment, and an attempt to disorient and make the prisoner feel terrified, isolated and helpless, as well as in potential danger. I would have no problems authorizing or doing such things myself. Our military and police departments in the US use such techniques routinely and legally. And they are often, but of course, not consistently, effective.

Anything that causes injury, or pain, is out of line. I think the distinction is pretty clear, although I recognize that there are a very few methods, such as mock executions, that might be considered borderline between the two extremes. These are the only judgement calls I can see.