Radical nationalism among veterans is nothing new, no, I'm not hinting at the Nazis, that would be a stupid comparison, but you get the idea. The US does not seem susceptible to a coup d'etat, but radicals among them may be desensitized to violence and more acutely aware of their own capabilities. They may be suffering from mental illness made worse by prolonged exposure to combat stress. Wasn't Timothy Mcveigh a veteran.
It's entirely possible for patriotism to get bent out of whack into something more sinister, especially if they feel betrayed by their leaders.