Typically, a wire or cord is attached to some device for detecting or reacting to physical movement. From this basic meaning, several extended and metaphorical uses of the term have developed. For example, the Berlin Brigade stationed in the divided city of Berlin during the Cold War was given the mission to be the "tripwire" for a Soviet incursion into West Germany.
Military usage may designate a tripwire as a wire attached to one or more mines — normally bounding mines and the fragmentation type — in order to increase their activation area. Alternatively, tripwires are frequently used in boobytraps, whereby a tug on the wire (or release of tension on it) will detonate the explosives.
Soldiers sometimes detect the presence of tripwires by spraying the area with Silly String. If the string falls to the ground there are no tripwires. If there is a tripwire, the string will be suspended in the air without pulling the wire. It is being used by U.S. troops in Iraq for this purpose.
Referred to in the movie Forrest Gump
Lieutenant Daniel Taylor: What's wrong with your lip?
Bubba: I was born with big gums, sir.
Lieutenant Daniel Taylor: Well, you better tuck that in. Gonna get that caught on a tripwire.
To conclude, I will again agree that we were a trip wire for World War III, but what a great tripwire we were. If an invasion had ever occurred we would have innevitably lost, but the price that the enemy wold have paid to take Berlin would have crippled them.