With the appearance of the legionary, the Roman army was able to maintain a vast empire that totally embraced the Mediterranean Sea. Although the success of the army rested on the backs of the foot-soldiers and cavalry, there were others on the field and in camp who enabled them to prevail. Besides the famed centurion who stood at the front of his cohort and led his legionaries into battle, there was a command hierarchy of military tribunes, a camp prefect, and a legate. Alongside the centurion in the thick of battle were the principales: optio, signifier, aquilifer, and tesserarius. There were others, some with specialized skills, who were as essential but remained in camp. These were the immunes and beneficiari: laborers, clerks, surveyors, architects, engineers, and orderlies. The legionaries could not have conquered and maintained an empire without this capable support; together they made the Roman army a feared adversary for over eight centuries.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/1711/officers-of-the-roman-army/.