A supply line is a large line of military supply vehicles, usually in convoy format. The line is useful for transporting needed supplies (food, medical supplies, ammo) to the front line. Without the supply line, a deployed army far from home would not be able to fight for long, as supplies would soon run out. The enemy is most likely to target the supply line if not heavily defended, since once the supplies are cut off, the army will soon be unable to fight, and unable to reinforce, and so they will be forced to surrender or die.