federate (v.)

1814 (implied in federated), a back-formation from federation, or else from Latin foederatus "leagued, federated, combined; having a treaty, bound by treaty," past participle of foederare "to establish by treaty," from foedus "covenant, treaty, alliance" (from PIE root *bheidh- "to trust, confide, persuade"). Related: Federating. As an adjective, by 1710.