The Federation has exacting requirements for prospective member worlds that wish to join. Caste-based discrimination is forbidden Accession, and major systematic violations of sentient rights, such as the unjust peacetime imprisonment of specially modified soldiers on the planet Angosia are also deemed serious impediments for acceptance. Furthermore, while most member worlds have single, unified world governments, it is not required for entry, as the Federation will consider associate membership of non-unified worlds. The Federation has come into military conflict at various times by the Klingon Empire, the Romulan Star Empire, the Tholian Assembly, the Cardassian Union, the Tzenkethi, the Borg Collective, the Dominion, the Breen Confederacy, and the Sphere Builders. Hostility has also existed with the Ferengi Alliance, the Sheliak Corporate, and the Talarian Republic, but it is unrevealed if any actual state of war with these species occurred, particularly the latter two. It seems, in fact, that the Federation has at one time or another been under hostile conditions with virtually every other major power in their vicinity.