Not true--what they are proscribed from doing is taking up arms against the U.S. or giving aid/comfort/assistance to enemies of the US. If the "civil war" is one in which the US is not involved, they can go fight in it any time they want (see Jewish US citizens fighting in the Israeli army, or the leftist "Lincoln Brigade" in the Spanish Civil War to pick just two examples).
Nope ! Many of those, who went to fight, in Spain, WERE punished, once they came home ! That is precisely the example I was using ... the Lincoln Brigade. LOL
Thanks for allowing me to correct you and prove you incorrect at the same time. : - )