Looper.com presented some interesting fan theories for various film characters but I found the following theory very interesting. The origins of the Joker are shrouded in mystery with no single decisive account of how he came to be. A poster on Reddit presented this idea a few years ago (not sure if they got it from somewhere else) and it makes a lot of sense. The Joker is very adept at planning and execution (!) along with the use of firearms and explosives. The scarring to his face (as well as his scarred psyche) could be as the result his experiences during war. It could also account for his lack of identity especially if he’s registered as KIA.