tripwire

(tripwīr′)

noun

a hidden wire that will set off a trap, snare, etc. when tripped on

See tripwire in American Heritage Dictionary 4

noun
  1. A wire stretched near ground level to trip or ensnare an enemy.
  2. A wire or line that activates a weapon, trap, or camera, for example, when pulled.
  3. A small military force whose involvement in hostilities will trigger the use of a larger force.

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