I'm a loyal member of the Democratic Party and I don't have a problem with Drones

[Recursion]

10. When I was in Iraq, there were always rumors of Americans fighting on the other side

I don't think there's much truth to that, but let's grant it for the sake of argument, because I think it's useful to draw out a distinction here.

If I, as a Marine, were on patrol and started taking fire from an American citizen who had decided to help the mujahideen, I seriously doubt there would be much argument that I could return fire, even if I somehow knew that was an American citizen, or that POTUS could order my unit to attack the village he was in. As far as that goes, I think it's pretty uncontroversial: that is an example of an American in arms against the US.

What we're talking about here is the President of the United States deciding that a particular American citizen, because of his actions, can be deliberately targeted for assassination. Not "targeted for arrest and things might get rough when we go get him, wink wink" (that happens in the US every day as it is): specifically targeted to be blown up. He may be giving aid an comfort to our enemies, but there's a huge difference between soldiers killing an American who is at the time of his death actively fighting them, and killing an American who is only "in combat" in the sense that we just launched a missile at him.