Across all forms of media, it's fairly common knowledge that The Joker's origins are a real grey area. The version seen in The Dark Knight was really no different in that regard, but one fairly well-known fan theory about the origins of Ledger's Joker suggests that he was a war veteran - most likely having served in Iraq - who was suffering terribly from PTSD, and his behaviour was as a result of that. It's backed up by several facts, including the fact that he clearly has the tactical skills of a man with combat training and experience. For example, he was easily able to infiltrate protected areas, had the ability to both acquire and create weapons, and he put together extremely intricate and dangerous plans with a high degree of success. More explicitly, he performed a highly regimental gun routine to perfection in the scene shown in the accompanying picture for this entry. He was quoted as saying you see, madness, as you know, is like gravity all it takes is a little push. That push could quite easily have been an experience in a war zone. It is also notable that he referred to himself as an agent of chaos. Maybe this was a reference to the fact that he really was an agent at some point perhaps a special-ops agent when he was a member of the American military.
