>>>If that is the defence, it looks a pretty solid one.<<<

No, not that he knew after the fact that they were conspirators. He knew (or should have known), while fighting on their behalf, that they were conspirators-after-the-fact (after the crime.) He knew (or should have known) they were harboring, aiding and abetting a criminal (and enemy of the State) after the criminal had been accused by the State. Not a good place to be. He took up arms against his countrymen to protect an accused criminal who has threatened to do more harm to Walker's countrymen. Bad, bad.