He brought up how the news and everyone freaks out when a boat full of people gonna die but says nothing when a truck full of men gets blown into pieces.
He understands strategic battle planning , psychology and interrogation more than just a average person or street criminal and has extensive knowledge of a wide range of firearms and explosives. If he was ex military he definitely wasn’t just standard infantry
I know this is basically impossible since Heath is gone now….but imagine a war story about a CIA SAD agent or SOF operator from Iraq and Afghanistan with some strange personality quirks going through all of the worst the US military could do to a uniformed soldier…and by the end when he is dishonorably discharged…we see the Joker begin to take over an eventually gaining Gordon’s attention at the very end.
What makes Ledger's Joker my favorite version of the character is that he's a master strategist/tactician. He's like the battlefield generals of old who commanded troops from the front line. He's usually several steps ahead of his adversaries, and when he isn't, he can adapt and improvise with remarkable expertise.
Not only he was likely in the military, I also think he had a fast and successful career, maybe even landing in CIA before becoming disillusioned and maybe even being betrayed.
Pet theory: The original scar was just one side, either in a crime or accident, but he did the other side to himself as some kind of statement that created the Joker persona and led secondarily to the getup. His simpatico moment with Two-Face would explain a lot, since Dent only has the one side.
He could definitely fit as a Green Beret, as their primary function is to infiltrate hostile nations, organize counter-militias, and train them in guerrilla warfare, which heavily relies on demolition and ambush tactics.
when Joker tells Gordon how alone he is, I think Joker knew Bats was behind him already. He wanted to have a face to face with Batman, size him up, learn things. It's the dinner scene from Heat.
The SpecOps angle seems to fit... especially the "in deep" ones that work closely with Intel Ops. This also explains how he could erase his identity down to the point of not even having traceable clothing