It is important to remember that actors earn their livings simulating emotion whilst mouthing the lines written for them by people who really did know what they mean. It is one reason writers so often despise actors. The money's another.
Mortenson is an idiot who understands neither the source material nor the current world situation. But at least he's consistent - what he's doing is parroting lines written for him by other people.
If the man was truely anti-war he would have turned down a part that glorfies it. He didn't.