The Collaborative International Dictionary of English v.0.48:

Federate \Fed"er*ate\, a. [L. foederatus, p. p. of foederare to
   establish by treaty or league, fr. foedus. See Federal.]
   United by compact, as sovereignties, states, or nations;
   joined in confederacy; leagued; confederate; as, federate
   nations.

   Syn: federated.
        [1913 Webster]

WordNet (r) 3.0 (2006):

federate
    adj 1: united under a central government [syn: federate,
           federated]
    v 1: enter into a league for a common purpose; "The republics
         federated to become the Soviet Union" [syn: federate,
         federalize, federalise]
    2: unite on a federal basis or band together as a league; "The
       country was federated after the civil war" [syn: federate,
       federalize, federalise]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "federate":
   act in concert, act together, affiliate, ally, amalgamate,
   associate, band, band together, be in cahoots, be in league, bunch,
   bunch up, cabal, cement a union, centralize, club, club together,
   coact, coalesce, collaborate, collude, combine, come together,
   concert, concord, concur, confederate, consociate, consolidate,
   conspire, cooperate, couple, do business with, federalize, fuse,
   gang, gang up, get heads together, get together, go in partners,
   go in partnership, go partners, hang together, harmonize,
   hold together, hook up, hook up with, join, join forces,
   join fortunes with, join in, join together, join up with,
   join with, keep together, league, league together,
   make common cause, marry, merge, organize, pair, pair off, partner,
   play ball, pull together, put heads together, reciprocate,
   stand together, stand up with, team up, team up with, team with,
   throw in together, throw in with, tie in, tie in with, tie up,
   tie up with, unionize, unite, unite efforts, unite with, wed,
   work together