to act on the offensive

from The Collaborative International Dictionary of English v.0.48

Offensive \Of*fen"sive\, n.
   The state or posture of one who offends or makes attack;
   aggressive attitude; the act of the attacking party; --
   opposed to {defensive}.
   [1913 Webster]

   {To take the offensive}, {To act on the offensive}, {To go on
   the offensive}, to be the attacking party; to initiate
      hostilities.
      [1913 Webster +PJC]