Triarii

Triarii were the third line of infantry in the Roman legion during the early to middle Republic. The name comes from Latin triarius, meaning the third or third rank. In the triplex acies structure—hastati in front, principes in the middle, and triarii at the rear—the Triarii formed the army’s veteran reserve and were typically the most experienced, and often older, soldiers.

Equipment and armament: Triarii fought primarily with long spears (hastae) and carried large shields (scuta). They wore body armor such as lorica hamata and used the gladius as a secondary weapon when close combat ensued. Their gear reflected their role as heavy, durable troops able to anchor a line under pressure.

Tactics and role: The Triarii were not usually deployed for early skirmishing. Instead, they waited in reserve to support the front lines if the battle turned dire. When the Hastati and Principes faltered, the Triarii would advance to stabilize the fight, potentially delivering the decisive effort that could restore Roman initiative. Their presence signaled a crisis and a willingness to endure heavy casualties to avoid a rout. The phrase ad triarios redisse—meaning to resort to the Triarii as a last resort—entered Latin literature to denote a final, often desperate, fallback.

Historical development: The Triarii were integral to the old Roman triplex acies system established in the early Republic and remained a formal element into the 2nd century BCE. Over time, reforms that reorganized legions into cohort-based structures under late Republican and imperial practice diminished the Triarii as a distinct operational unit. Nevertheless, the concept persisted as a symbol of a last-ditch, veteran reserve in Roman military thinking.