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
同义词：trip wire
a wire stretched close to the ground that activates something (a trap or camera or weapon) when tripped over
同义词：trip wire
英汉双解大词典
tripwire/ˈtrɪpˌwaɪə/(tripwires)
1.
N-COUNT（可数名词）
A tripwire is a wire stretched just above the ground, which sets off something such as a trap or an explosion if someone touches it. (如陷阱或爆炸装置的)绊线