Would it have been possible at any time, for the US to form a French style foreign legion? Maybe during a period of isolationist sentiment, a foreign legion is formed to allow the US to wage foreign wars without risking US lives. The USFL would be headquartered outside CONUS to eliminate the possibility of a coup, and all recruits must assume a non de guerre. Just like in France, those who successfully serve say 5 years in the foreign legion are offered citizenship.
Could it start off as an end run around isolationism, and gradually evolve into a celebrated part of the US military? And, would the existence of a US foreign legion change the debate around immigration ("people from all over the world come here to fight under our flag, so we should welcome them").