NOUN (2)

1. a small military force that serves as a first line of defense; if they become engaged in hostilities it will trigger the intervention of stronger military forces;

2. a wire stretched close to the ground that activates something (a trap or camera or weapon) when tripped over;

WordNet (r) 3.0 (2006):

trip wire
    n 1: a small military force that serves as a first line of
         defense; if they become engaged in hostilities it will
         trigger the intervention of stronger military forces
    2: a wire stretched close to the ground that activates something
       (a trap or camera or weapon) when tripped over