tripwire

A wire stretched near ground level to trip or ensnare an enemy.

A wire or line that activates a weapon, trap, or camera, for example, when pulled.

A small military force whose involvement in hostilities will trigger the use of a larger force.

A cord or wire arranged so that when snagged or pulled by an intruder, it will trigger a detector or trap or a device, such as a land mine.

Any means of detecting intruders.