Size of the army

Not until the testimony of John the Lydian is any kind of overall number available for the size of the Roman army. Estimates of its strength before the reign of Diocletian must rest on a combination of scanty literary testimony – practically limited to the already mentioned ambiguous sentence from Tacitus Annales IV.5 – usually interpreted to say that the total of auxiliary troops was about equal to the total number of legionaries. A better total could perhaps be constructed by counting the auxiliary units which figure in diplomas, dedications, epitaphs and other inscriptions. That would require clarity in respect to the naming of every unit however, something which is often not the case. For now it may suffice to suppose that the evolution of the size of the Roman army can be traced from the development of legionary strength. As the lasting core component of the emperor’s armed forces, it may be expected that other units more or less kept pace. When reviewing that development, it is immediately striking that it is anything but a linear progression. Augustus’ 28 legions were reduced to 25 by the Varian disaster and were not reinforced for another thirty years when Caligula added two new legions for his planned expedition to Britain. Between 66 and 70 seven new legions were added, while six disappeared in the turmoil of the civil war and the Batavian revolt. The total was only now back at the Augustan 28. Domitian’s reign saw the disappearance of one legion and the foundation of a new one. Trajan finally raised the sum to thirty legions, which remained the norm for most of the second century. Septimius Severus really expanded the army by adding the three Legiones Parthicae, while it is also known that he considerably enlarged the Rome units. Finally, it seems likely that Alexander Severus again raised the legionary total, this time to 34 with IV Italica. Over the course of some 250 years, legionary strength increased by 20 percent for an enormously expanded empire in which, for instance, one new province – Britannnia – needed three or even four of those six legions. With such territorial commitments, it is perhaps not surprising that emperors were loathe to transfer entire legions elsewhere considering the dangers in the area they had left. The solution to this problem may have been one of the reasons for the ascendancy of the campaign armies of the third century onwards.