Both treachery and treason imply a willful, deliberate betrayal of trust or confidence. Treachery could be applied to the act of being disloyal to a friend or to making unkind statements about someone behind his back. Treason, however, applies solely to betrayal of one's country, to disloyalty to one's citizenship, to violation of allegiance to one's chosen land: "Benedict Arnold committed an act of treason." All treasonable acts are treacherous, but not all treachery is treason.
Dictionary of problem words and expressions. Harry Shaw. 1975.