During WWII many American citizens went to Germany to fight for Hitler. When captured and discovered to be Americans they were often summarily executed as traitors by military tribunals. This was and is entirely legal by a hundred years or more worth of international rules of war.
A few years back the Rats tried to claim that US citizens who joined alQaeda were entitled to lawyers and civil trials. This is completely false. However corrupt left wing pols ignored the law and used their false claim simply for another hammer to beat Bush with. Obama was one of these pols. Now he gets to deal with an issue that he helped to create which is too bad for him.
But it is completely legal to kill US Citizens who take up arms in foreign lands against the USA. They can be killed on the field or sleeping in their beds or anywhere any time unless they surrender at which time they are subject to military jurisdiction. Ron Paul and the ACLU are completely wrong.
If aiding and abetting an undeclared "enemy" is sufficient cause for the U.S. government to take this kind of action, then I fail to see how Jane Fonda can go about her business freely in this country to this very day.