The Empire of Thrace was an interstellar political entity, headed by the Matriarch, and had established contact with the United Federation of Planets by the 23rd century. The Empire was independent of both the Federation and the Klingon-Romulan Alliance, but often welcomed visitors from both powers and the Empire's home planet, Macedon III, was considered to be an excellent liberty port. (TOS - Star Trek II Short Stories short story: "Under Twin Moons")