I checked all the way through the 616-page collectors' reference-book "American Military Belt Plates" for you, and your buckle isn't in that book, unless I somehow missed seeing it. Your buckle has the characteristics of a Militia waist-belt plate, from sometime between the 1850s through about 1900. The fact that it is a semi-thick "rolled-brass" plate with brazed-on attachment provisions and a stamped Inspector's-mark "M" on it strongly indicate it is an Original, not a Reproduction. That's all I can tell you about it from its form-&-construction.
It shows a typical Militia-style emblem of the mid-1800s. It might be South Carolina, because the "tree" somewhat resembles a Palmetto tree. Atop the tree is a "Liberty Cap," another common Militia symbol. The tree is flanked on each side by various weapons and draped flagpoles. all above two cannons facing opposite directions. The motto on the ribbon is too worn for me to read it. If you could tell us what it says, perhaps by reading the back of the buckle, we could research the motto.