How were officers trained?

[Theodosius the Great]

Officers, it seems, simply learned by osmosis. From Republican times all the way into the Later Roman Empire we read time and again that aristocrats would take their sons on campaign from an early age (after receiving the Toga virilis ). They would learn from their fathers and from the other officers as well just by being around them if nothing else.

Some well known examples :

• Scipio Africanus, still in his teens, campaigned with his father.
• Marcus Lincinius Crassus took his son Publius on campaign in Parthia where they both perished
• Vaspasian (as general) appointed his son, Titus, as a legate during the Jewish war
• Constantius Chlorus always had Constantine by his side from early on
• Theodosius the Elder (aka "Count") brought his son with him during the campaign to reconquer Britain

~Theo