The point is no US military veteran with a brain would have gone from say, the Special Forces, to Blackwater in Iraq, if he was going to fall under Iraqi law. You'd be crazy to place yourself in that kind of jeopardy.
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For the chance to strut about heavily armed among cowering Iraqis and the sort of salary Blackwater was paying, I think some degree of craziness might emerge.
It appears that Blackwater's mercenaries were accountable to no one other than Blackwater itself, which only had the power to fire them.
Still, there should have been more accountability than that, though it appears that this was not the case. Rumsfeld was a man whose incompetence was unmatched except for his towering ego.