noun
defense line; line of defense; enemy lines
Used for military defense lines, but also metaphorically for any defensive boundary or last line of defense.
敵の防衛線を突破する作戦が立てられた。
A plan was made to break through the enemy's defense line.
この堤防は洪水から街を守
This levee is the last line of defense protecting the town from floods.
防御線 is a near-synonym also meaning 'defense line', but 防衛線 is more common and often used in broader contexts including metaphorical ones.
Compound of 防衛 (defense) + 線 (line).