forward edge of the battle area
Military term for the front line of a defensive position; used in tactical contexts.
The enemy is approaching the forward edge of the battle area.
前線 is a more general term for 'front line' in both military and non-military contexts, while 戦闘陣地の前縁 is a specific tactical term.
Compound of 戦闘陣地 (battle position) and 前縁 (forward edge), a direct translation of the English military term.