A legionnaire is also a member of a legion, but English kept this French-shaped word for later, living forces — above all the French Foreign Legion, where soldiers from many countries serve France. Members of the American Legion, a US veterans’ group, are often called Legionnaires too. So legionary leans old Rome; legionnaire leans modern, especially French, and sometimes US veterans. You may also hear Legionnaires’ disease, a serious lung illness named after an outbreak at an American Legion meeting. That borrowed the name; it is not a meaning of the soldier word itself.