scouting line; outpost line
Military term for a line of forward positions used for reconnaissance or early warning. Not an everyday word.
敵の前哨線を突破する作戦が立てられた。
A plan was made to break through the enemy's scouting line.
general outpost line
Compound of 前哨 (outpost, advanced guard) and 線 (line). The exact historical derivation is uncertain, but the word follows standard Sino-Japanese military terminology patterns.