noun
frontline troops
Refers to military units stationed at or engaged in the front line of a battle.
前線部隊が敵の攻撃を食い止めた。
The frontline troops held off the enemy attack.
前線 means 'front line' (the line itself), while 前線部隊 specifically refers to the troops stationed there.
Compound of 前線 (front line) and 部隊 (unit, corps).