I will focus on the roman army because is one of the topics of my research. Veterans made a difference in various aspects. The first one, was in terms of cohesion. Nowadays, ancient battles tend to be interpreted as a serious of charges and mostly focused in the exchange of projectiles until one of the contenders broke formation. Authors that discuss in extension this are Sabin or Zhmodikov. The moment part of the army was in disarray is when is more susceptible to suffer casualties and, consequently, start routing. In this moment, is when the highest amount of casualties took place. Therefore, the ability of the soldiers to withstand these moments is vital to the survival of the troops.

The other factor was in attacking the enemy. Veteran soldiers know and have more experience and therefore have a highest impact on the battlefield. For example, in Pharsalos you can see how the Caesar soldiers maintain their position when the pompeians charge, even when the stop in the middle of the charge. Caesar pounders how the pompeians made a mistake as they lost the impetus. But this also can be seen in the ability of the soldiers to maintain cohesion doing some elaborate deployments during the battlefield. For example, take the movement made by Scipio troops during the battle of Ilipa.

Some years ago, with some colleagues, we created a computer simulation to study the importance of the centurion in the roman army. In it, we could see how important was to maintain cohesion of the troops when the soldiers have very little experience. The more experience the centurion had, better results. In the other hand, a centurion paired with veteran troops changed where it was most effective, increasing the impact of the troops in the attack. We concluded that the way the centurion was portrayed in Polybius and Cesar was a reflection of the armies of their era, with Polybius seeing armies with many recently recruited troops and Caesar having a very veteran army. Therefore, the centurions played roles extremely different.

Bibliography

Rubio, Xavier, Pau Valdés Matías, y Eduard Ble. 2015. «Centurions in the Roman Legion: Computer Simulation and Complex Systems». Journal of Interdisciplinay History XLVI (2): 245-63. https://doi.org/10.1162/JINH_a_00833.

Sabin, Philip. 1996. «The mechanics of battle in the Second Punic War». En The Second Punic War: A reappraisal, editado por Tim J. Cornell, Boris Rankov, y Philip Sabin, 49-57. Bulletin of the Institute of Classical Studies 41. University of London: Institute of Classical Studies, School of Advanced Study.

Sabin, Philip. 2000. «The Face of Roman Battle». Journal of Roman Studies 90: 1-17.

Zhmodikov, Alexander. 2000. «Roman Republican Heavy Infantrymen in Battle (IV-II centuries BC)». Historia: Zeitschrift für Alte Geschichte 49 (1): 67-78.