My little Republican Roman army has been left without update for some time now and they have not really got one unit they should have had from the beginning. Triarii tend to be on very expensive in Impetus but I opted to have two units. Three ranks would look much, much better than two but I thought that two ranks would be easier to distinguish. Really, I should have them in three. EI4 has updated Republican army list where concept of mandatory legion purchase is finally abolished and it makes sense to have them around sometimes.
In early days, Romans organized themselves same way as Etruscans had done – in phalanx formation. This came into abrupt and in the Battle of Allia where most curious disaster occurred. Romans had in their center the wealthiest, best equipped and oldest men while both flanks were composed of poor and poorly armed conscripts. It is anyones guess what happened next. Following Gallic onslaught, the flanks fled, leaving the center surrounded and annihilated. Resulting loss of many leading citizens called for military reform, and Camillus quickly put it in action.
Obvious answer to the disaster was then to rank men by wealth. Velites, the poorest acted as javelin armed skirmishers. Hastati, younger men, and Principes those in prime of their lives and with heavier equipment were the real fighting force. Triarii, the wealthiest of citizens were then posted in the rear and were only employed if both before failed. It is doubtful that role of Triarii was solely to fight as a third line, but also to ensure the integrity of the two in front.
System where wealthiest were the last line of battle or cavalry reserve in form of Equites was obviously very good for the wealthier population as long as the preceding lines held, and no extraordinary tasks were demanded from Equites cavalry. Once they didn’t, the drawback of the system became evident and reformation took place. This happened when Rome met Hannibal during Second Punic War. Another reform was called for, and this time it was turn of Polybius.
Polybius choose to arrange the ranks by experience. Essentially system remained same (albeit distinct rorarii (reserve line) and accensi (light armed third line), were dropped altogether) but Triarii consisted of the most experienced and battle hardened men in the legion. While system worked, it was too late to live for very long. Even inexhaustible Roman manpower pool started to dwindle under pressure from north and south. Marian reform after Hannibal’s Great Italian Tour 107 BC scrapped whatever was left of the class system and military service became a career available for all. That spelled the end for Triarii.