8 Theories About The Origins Of Heath Ledger’s Joker
8. Hes A Former Soldier
One of the more common theories about the Joker's origins, this is primarily sparked by the throwaway line on chaos, "if, tomorrow, I tell the press that ... a truckload of soldiers will be blown up, nobody panics, because it's all part of the plan." Rather than an example plucked out of thin air (like the gang banger getting shot) this is actually a clue towards where Joker came from; he is a soldier with PTSD.
Obviously there's no real timeline, but it'd be believable he was a solider in one of America's middle-eastern conflicts who at some point became psychologically damaged by his experiences and grew to hate the futility of modern society (think of the opposite journey Bruce Wayne goes through in the opening of Batman Begins).
It really ties into the film's post-9/11 world view, having the toughest person Batman has gone up against become a product of the time. And, on a more basic level, it serves as a logical explanation of many facets of his character. How he's able to handle multiple types of weapons with ease makes a lot more sense, while it's easy to imagine the horrors of war leading towards his deranged psychological state. The only thing it doesn't really explain is the chelsea smile.