Roman legion
The triarii (sing. triarius) were the veteran soldiers, to be used in battle only in extreme situations; they rested one knee down when not engaged in combat. The triarii served primarily as reserves or barrier troops to backstop the hastati and principes. They had long hastae (spears) rather than the pilum and gladius. Thus armed, they fought in a phalanx formation. The sight of an advancing armored formation of triarii legionaries frequently discouraged exultant enemies in pursuit of retreating hastati and principes troops. To fall upon the triarii was a Roman idiom – meaning to use one's last resort.