Triarii

Triarii (singular: Triarius) were one of the elements of the early Roman military manipular legions of the early Roman 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.

During the Camillan era, they fought in a shallow phalanx formation, supported by light troops. In most battles triarii were not used because the lighter troops usually defeated the enemy before the triarii were committed to the battle. They were meant to be used as a decisive force in the battle, thus prompting an old Roman saying: 'It comes down to the triarii' (res ad triarios venit), which meant carrying on to the bitter end.

According to author Pat Southern, triarii may have evolved from the old first class of the army under the Etruscan kings. The first class comprised the richest soldiers in the legion who were equipped with spears, breastplates and large shields, like heavy Greek hoplites. They served as heavy infantry in the early Roman army, and were used at the front of a very large phalanx formation. After a time, engagements with the Samnites and Gauls appear to have taught the Romans the importance of flexibility and the inadequacy of the phalanx on the rough, hilly ground of central Italy.

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 the Battle of the Allia.

This crushing defeat prompted a series of military reforms by Marcus Furius Camillus. Under the new system, men were sorted into classes according to wealth, the triarii being the richest after the mounted equites.Triarii were armed with spears, or hastae, about 2 metres (6½ feet) long. They also carried swords, or gladii, about 84 centimetres (29 inches) long, in case the spear broke or the enemy drew too close.

...
Wikipedia