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Constantius died at York in 306, after a successful campaign against the barbarous Picts north of the Wall. His son Constantine was proclaimed “Augustus” by the British legions, but a period of complicated wars, negotiations, bandying of titles and dynastic marriages intervened before Constantine (known to history as “the Great”) attained the supremacy which Diocletian, for all his fourfold system of government, had really enjoyed. The fourfold system was, in fact, among the less durable of Diocletian institutions, and its obvious vulnerability to the old maladies saddened him before his death in 316.
Constantine developed the Roman army along lines which Diocletian had laid down and which had been apparent even in Aurelian’s time. Static frontier forces (limitanei) occupied forts in peripheral zones or manned the lines of river barriers. The best troops, however, were reserved as a mobile striking force (comitatenses) which could direct its energies as emergency required. The infantry units of this striking force were still termed legions, though their strength was reduced to about a third of the old Marian legion. It would seem, in fact, that the original legions had sometimes been split and apportioned between the frontier garrisons and the emperor’s mobile field armies. The mobile forces, of course, had more need 9f a strong cavalry element, but Rome had long been accustomed to rely on the barbarians who were settled in frontier areas to supply cavalry, such forces having being classed as numeri.. There was a natural tendency, in the interests of security, to keep the barbarians on the frontier, away from the heart of the Empire, but in view of commitments to central mobility it could not indefinitely be upheld.
Constantine made one change which is symptomatic rather than important: he abolished the Praetorian Guard. The Praetorian cohorts were by now wholly redundant. Both their uses and abuses had been usurped by other sections of the army. The title of Praetorian Prefect was applied by Constantine to a purely civil official.
Constantine’s most monumental work was, of course, his building of Constantinople, the “New Rome” and second capital of the Empire. For this role the site of the ancient Greek city of Byzantium was chosen. A glance at the map will immediately make clear the economic and strategic importance of the position selected, at the centre of land communications between Europe and Asia and of sea communications between the Mediterranean and the Black Sea. Above all, Constantinople was ideally placed as a general headquarters for operations on the all-important Danube front.
Septimius Severus, with something less than his usual foresight, had destroyed Byzantium after the city supported his eastern rival Pescennius Niger. As a result, there remained no effective base or stronghold against the Goths, who in the following generation commandeered the fleets of Greek Pontic cities and swept down in their piratical raids into the Aegean Sea. In building the walls of his new capital, Constantine was affirming his faith in fortifications in general and in the importance of fortifying this particular point. Constantine’s fortifications are not those which now survive, but the position was eminently fortifiable. The barbarian invaders were never very successful in attacking fortified cities, and the walls of Constantinople withstood their attacks throughout many centuries to come.
Constantine is perhaps best known as being the first Christian emperor. In fact, he became a Christian on his death-bed, but before that date had, like other imperial pretenders of his generation, given support and encouragement to the Christians. The most immediate and tangible military effect of his attitude was the adoption of Christian battle standards. These featured a monogram compounded of the first two letters of Christ’s name in Greek (XP). Constantine also had the device painted on his soldiers’ shields, and it was first carried into battle when, in 312, he invaded Italy, to wrest power from Maxentius, son of Diocletian’s old colleague Maximian.
