From this passage of Titus Livius, then, we learn that the Roman army had three principal divisions, or battalions as we might now call them, of which they named the first hastati, the second principes, and the third triarii, to each of which cavalry were attached. In arraying an army for battle they set the hastati in front. Directly behind them, in the second rank, they placed the principes; and in the third rank of the same column, the triarii. The cavalry of each of these three divisions they disposed to the right and left of the division to which it belonged; and to these companies of horse, from their form and position, they gave the name wings (alae), from their appearing like the two wings of the main body of the army. The first division, the hastati, which was in front, they drew up in close order to enable it to withstand and repulse the enemy. The second division, the principes, since it was not to be engaged from the beginning, but was meant to succour the first in case that were driven in, was not formed in close order but kept in open file, so that it might receive the other into its ranks whenever it was broken and forced to retire. The third division, that, namely, of the triarii, had its ranks still more open than those of the second, so that, if occasion required, it might receive the first two divisions of the hastati and principes. These divisions, therefore, being drawn up in this order, the engagement began, and if the hastati were overpowered and driven back, they retired within the loose ranks of the principes, when both these divisions, being thus united into one, renewed the conflict. If these, again, were routed and forced back, they retreated within the open ranks of the triarii, and all three divisions, forming into one, once more renewed the fight, in which, if they were overpowered, since they had no further means of recruiting their strength, they lost the battle. And because whenever this last division, of the triarii, had to be employed, the army was in jeopardy, there arose the proverb, “Res redacta est ad triarios,” equivalent to our expression of playing a last stake.
The captains of our day, as they have abandoned all the other customs of antiquity, and pay no heed to any part of the ancient discipline, so also have discarded this method of disposing their men, though it was one of no small utility. For to insure the defeat of a commander who so arranges his forces as to be able thrice during an engagement to renew his strength, Fortune must thrice declare against him, and he must be matched with an adversary able three times over to defeat him; whereas he whose sole chance of success lies in his surviving the first onset, as is the case with all the armies of Christendom at the present day, may easily be vanquished, since any slight mishap, and the least failure in the steadiness of his men, may deprive him of victory.