Organisation

The combined legions of the Julio-Claudian dynasty formed, combined with their auxiliaries, campaign armies, always poised to take the fight into unconquered areas. Whenever war or an operation was planned, entire legions were uprooted and moved elsewhere. Some went to the theatre of war, while others were redeployed to new garrisons. Central coordination of whatever kind must have been involved in this process to maintain a balanced defence all over the empire. Gradually however, this policy started to change. Domitian put a stop to the policy of putting two legions in the same camp to prevent sedition. Instead, the legions were more spread out across the provinces they watched over. The balance of forces was altered as well, shifting gradually east. More and more legions would be based in the Balkans and the east, to the detriment of Spain and both German provinces. From the early second century the disposition of the legions seems to have become fixed. Once a garrison had been taken up, the core of the legion hardly ever moved away. Local recruitment meant strong local ties and increasing pressure on the borders meant that moving entire legions away had simply become too dangerous. From the reign of Hadrian onwards, whenever a campaign army needed to be gathered, legions would send so called vexillationes, consisting of several cohorts under a praepositus to the campaign army. This would then return to its original legion after the completion of operations. The continuous warfare on the Danube in the reign of Marcus Aurelius made the drawing off of vexillations from legions virtually a permanent feature. The former became almost independent units, sometimes even called legions. Auxiliary units and the fleets – to a lesser degree – passed through similar developments.