Yes, IowaRelic is correct... your button is from the early-1800s. More specifically, it is what relic diggers call a 1-piece (not hollow 2-piece) brass "flatbutton." On that particular type of button, the backmark being written in indented (not raised) lettering indicates it was made sometime between about 1810 and the late-1830s. Also, your button's backmark says "Warranted," which is archaic British for guaranteed, thus indicating your button was made in Britain and imported into the US in the time before the young American button-making industry became capable of mass-producing metal buttons.
By the way, because you might be wondering... although a few of these blank-front 1-piece brass flatbuttons did get used on Militia uniforms, they were manufactured for use on Civilian coats & jackets, and as pants-waist & fly closure buttons. (Shirts did not require such "sturdy" metal buttons, so wooden or ceramic/glass buttons were used on shirts.)