I am looking for ways to determine if Civil War soldiers were hired mercenaries. I know the name of my great-great grandfather only from a census record when he was living with his son. From that 1880 census I also have his approximate age. From city directories I know he lived in Philadelphia from 1875- 1891. His son Emanuel (my great-grandfather), when being interviewed after being awarded a silver star, told stories to the reporter of "his grandfather who fought against Napoleon at Waterloo, or of his father who fought for the Union under Lincoln and later ag[ai]nst France under Bismark." (From newsclipping). Emanuel told those same stories to my father as a child. These are my only hints to find my great-great grandfather, or anyone before him in that line.
Supposing the war stories told by my great-grandfather about his father are true, how could I go about finding a Frederick Ziegler who fought in the Civil War (there were many) from Wurtemburg (still quite a few) but who was a paid mercenary who returned to Germany to fight in the Franco-Prussian War (1870-1871)? While it is possible he immigrated and joined the army right off the boat, it seems more likely that he was a paid mercenary because he returned (according to the stories) to Germany to fight in the Franco-Prussian War. Any suggestions?