Actually, the backmark says A.D. Et Cie ...which is French for A.D. and Company.
Just in case you don't already know... the backmark made in the style which button-collectors call RMDC (Raised-Mark-in-a-Depressed-Channel). On a two piece button, that style of backmark tends to mean the button dates from the 1820s through the 1860s, although there are a few later exceptions.
Unfortunately, its approximate manufacturing era and country-of-origin is all the "solid" information I can give you. It's interesting that its front shows a sword stuck into the face of a Roman soldier's helmet, surrounded by an Oak branch and a Laurel branch (which symbolizes Victory). Perhaps your button commemorates a historic victory of the Gauls against the Roman army.
