Triarii (sg.:triarius) ("the third liners") were one of the elements of the early Roman militarymanipular legions of the earlyRoman Republic (509 BC – 107 BC). They were the oldest and among the wealthiest men in the army and could afford high quality equipment. They wore heavy metal armor and carried large shields, their usual position being the third battle line. They were equipped with spears and were considered to be elite soldiers among the legion.^{[1][better source needed]}

During theCamillan era, they fought in a shallowphalanx formation, supported by light troops. In most battlestriarii were not used because the lighter troops usually defeated the enemy before thetriarii were committed to the battle. They were meant to be used as a decisive force in the battle, thus prompting an old Roman saying:res ad triarios venit, 'it comes down to the triarii', which meant carrying on to the bitter end.^[2]

According to authorPat Southern,triarii may have evolved from the old first class of the army under theEtruscan kings.^[3] The first class comprised the richest soldiers in the legion who were equipped with spears, breastplates and large shields, like heavy Greekhoplites. They served asheavy infantry in the early Roman army, and were used at the front of a very largephalanx formation. After a time, engagements with theSamnites andGauls appear to have taught the Romans the importance of flexibility and the inadequacy of the phalanx on the rough, hilly ground of central Italy.^[4][5]

By the 4th century BC, the military formations the Romans had inherited from the Etruscans were still in use. Though their efficiency was doubtful, they proved effective against Rome's largely local adversaries. When Gauls invaded Etruria in 390 BC, the inhabitants requested help from Rome. The small contingent Rome sent to repel the Gallic invaders provoked a full-scale attack on Rome and the entire Roman army was destroyed at theBattle of the Allia.^[6]

This crushing defeat prompted a series of military reforms byMarcus Furius Camillus. Under the newsystem, men were sorted into classes according to wealth, thetriarii being the richest after the mountedequites.^[6]Triarii were armed with spears, orhastae, about 2 metres (6½ feet) long. They also carriedswords, orgladii, about 84 centimetres (29 inches) long, in case the spear broke or the enemy drew too close.^[6]

They fought as hoplites, usually carryingclipei, large roundGreek shields, and wearing bronzehelmets, often with a number of feathers fixed onto the top to increase stature. Heavy plate armour was favoured, withmail also being popular.^[6] Many would paint or engrave portraits of ancestors onto their shield, believing that it would bring them luck in battle.^[7]

In this new type of unit, the 900triarii formed 15maniples, military units of 60 men each, which were in turn part of 15ordines, larger units made up of a maniple oftriarii, a maniple ofrorarii and a maniple ofaccensi.^[8] Thetriarii stood in the third line of the legion, behind the front line ofhastati and the second line ofprincipes, and in front of therorarii andaccensi.^[6] In apitched battle, theleves, javelin-armedskirmishers who were attached to maniples ofhastati, would form up at the front of the legion and harass the enemy with javelin fire and cover the advance of thehastati,spear-armed infantry.^[6]

If thehastati failed to break the enemy, they would fall back and let theprincipes, heavier and more experienced infantry, take over. If theprincipes did not break them, they would retire behind thetriarii, who would then engage the enemy in turn—hence the expressionrem ad Triarios redisse, "it has come to thetriarii"—signaling an act of desperation.^[6] Theequites, cavalrymen, were used as flankers and to pursue routing enemies. Therorarii, the poorer reserve soldiers, andaccensi, the least dependable troops armed withslings, would be used in a support role, providing mass and supporting wavering areas of the line.^[7]

By the time of theSecond Punic War of the late 3rd century BC, this system had proved inefficient against enemies such asCarthage. After a series of more "organic" changes as opposed to a single intentional reform, a new system gradually came into being. Infantry were sorted into classes according to age and experience rather than wealth, thetriarii being the most experienced.^[9] Their equipment and role was very similar to the previous system, except they now carriedscuta, large rectangular shields that offered a greater degree of protection than the old roundclipeus.^[10]

The number oftriarii were reduced to 600 per legion, forming 10 maniples of 60 men each.^[11] Thetriarii still made up the third line in the legion, behind the front line ofhastati and the second line ofprincipes, but therorarii andaccensi were phased out.Leves had been replaced withvelites, who had a similar role but were also attached toprincipes andtriarii.^[11][12]

Pitched battles were conducted in a similar fashion: thevelites would gather at the front and fling javelins to cover the advance of thehastati. If thehastati failed to break the enemy, they would fall back on theprincipes, who along with the hastati, had been re-equipped with pila rather than spears. If theprincipes could not break the enemy they would retire behind thetriarii, who would then engage the enemy.^[13]

This order of battle was almost always followed, theBattle of the Great Plains and theBattle of Zama being among the few notable exceptions. At the Great Plains,Scipio, the Roman general, formed his men up in the usual manner, but once thehastati had begun to engage the enemy, he used hisprincipes andtriarii as a flanking force, routing the opposing Carthaginians.^[14][15] At Zama, Scipio arranged his men into columns, side-by-side, with large lanes in between. The opposing Carthaginian elephants were drawn into these lanes where many were killed byvelites without inflicting many casualties on the Romans. Once the surviving elephants had been routed, Scipio formed his men into a long line with histriarii andprincipes in the centre andhastati on the flanks, ready to engage the Carthaginian infantry.^[15]

With the putative militaryreforms ofGaius Marius in 107 BC, implemented to combat a shortage of manpower due to wars againstJugurtha in Africa and Germanic tribes to the north, the different classes of units were scrapped entirely.^{[16][failed verification]}Auxiliaries, local irregular troops, would fulfill other roles, serving asarchers,skirmishers andcavalry.^{[17][obsolete source]} Sallust, in hisJugurthine War, describes several instances in which Roman or allied regular heavy infantry were equipped with light equipment and used as light footsoldiers.^[18][19] This was supposedly a common practice.

• List of Roman army unit types