The Collaborative International Dictionary of English v.0.48:

Ally \Al*ly"\, v. t. [imp. & p. p. Allied; p. pr. & vb. n.
   Allying.] [OE. alien, OF. alier, F. alier, fr. L. alligare
   to bind to; ad + ligare to bind. Cf. Alligate, Alloy,
   Allay, Ligament.]
   1. To unite, or form a connection between, as between
      families by marriage, or between princes and states by
      treaty, league, or confederacy; -- often followed by to or
      with.
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            O chief! in blood, and now in arms allied. --Pope.
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   2. To connect or form a relation between by similitude,
      resemblance, friendship, or love.
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            These three did love each other dearly well,
            And with so firm affection were allied. --Spenser.
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            The virtue nearest to our vice allied. --Pope.
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   Note: Ally is generally used in the passive form or
         reflexively.
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The Collaborative International Dictionary of English v.0.48:

Allied \Al*lied"\, a.
   United; joined; leagued; akin; related. See Ally.
   [1913 Webster]

WordNet (r) 3.0 (2006):

allied
    adj 1: related by common characteristics or ancestry; "allied
           species"; "allied studies"
    2: of or relating to or denoting the Allies in World War II; "an
       Allied victory"; "the Allied armies"
    3: of or relating to or denoting the Allies in World War I; "an
       allied offensive"; "the Allied powers"
    4: united in a confederacy or league [syn: allied,
       confederate, confederative]
    5: joined by treaty or agreement