The Collaborative International Dictionary of English v.0.48:

Guarantee \Guar"an*tee`\, v. t. [imp. & p. p. guaranteed; p,
   pr. & vb. n. Guaranteeing.] [From Guarantee, n.]
   In law and common usage: to undertake or engage for the
   payment of (a debt) or the performance of (a duty) by another
   person; to undertake to secure (a possession, right, claim,
   etc.) to another against a specified contingency, or in all
   events; to give a guarantee concerning; to engage, assure, or
   secure as a thing that may be depended on; to warrant; as, to
   guarantee the execution of a treaty.
   [1913 Webster]

         The United States shall guarantee to every State in
         this Union a republican form of government.
                                                  --Constitution
                                                  of the U. S.
   [1913 Webster]