Hello from France! I suppose your ancestor was made a prisoner or, kind of, deserted, and then decided it was better to stay where he was than to come back into what was left of France. Being in the army at 45, I suppose he was an officer or sub-officer. If an officer, the French Service historique de la défense could have a file: http://www.servicehistorique.sga.defense.gouv.fr/http://www.servicehistorique.sga.defense.gouv.fr/ If not an officer, or if no file, you're lost if you don't know his birth/residence place in France and/or the regiment he was in (on the mentioned site, look at "aide à la recherche" then "fiches thématiques" then the two pages "personnel de l'armée de terre depuis la Révolution"). It's all in French, of course. Please ask if you need more information.