on the alert

from The Collaborative International Dictionary of English v.0.48

Alert \A*lert"\, n. (Mil.)
   An alarm from a real or threatened attack; a sudden attack;
   also, a bugle sound to give warning. "We have had an alert."
   --Farrow.
   [1913 Webster]

   {On the alert}, on the lookout or watch against attack or
      danger; ready to act.
      [1913 Webster]