The Collaborative International Dictionary of English v.0.48:

Militate \Mil"i*tate\, v. i. [imp. & p. p. Militated; p. pr. &
   vb. n. Militating.] [L. militare, militatum, to be a
   soldier, fr. miles, militis, soldier.]
   To make war; to fight; to contend; -- usually followed by
   against and with.
   [1913 Webster]

         These are great questions, where great names militate
         against each other.                      --Burke.
   [1913 Webster]

         The invisible powers of heaven seemed to militate on
         the side of the pious emperor.           --Gibbon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

militate
    v 1: have force or influence; bring about an effect or change;
         "Politeness militated against this opinion being expressed"