Well, the same argument could be made about the Southerners who took up arms against the Union. Lincoln waged an undeclared war against the seceding states because he did not accept the way they renounced their citizenship. Technically, Confederate soldiers, as far as that tyrant Lincoln was concerned, could have been tried for treason but they weren't. The charges of treason were even dropped against Lee and Davis. The feds knew they could never make those charges stick so they dropped them. I think we'll see the same thing happen here. The only way out for Bush might be to claim that Walker is no longer a citizen and send him to one of his military tribunals. However, I wouldn't want to get caught on that slippery slope.
What's the process for doing that? What form? And you can't prove he faught against U.S. Troops.