to be on the defensive

from The Collaborative International Dictionary of English v.0.48

Defensive \De*fen"sive\, n.
   That which defends; a safeguard.
   [1913 Webster]

         Wars preventive, upon just fears, are true defensives.
                                                  --Bacon.
   [1913 Webster]

   {To be on the defensive}, {To stand on the defensive}, to be
      or stand in a state or posture of defense or resistance,
      in opposition to aggression or attack.
      [1913 Webster]