The Tetrarchy and the Wars of Constantine

Diocletian: The Reorganized War284–305

The reorganization the last volume ended by announcing was executed across twenty-one years with a system-builder's patience, and its military record — obscured by the reign's greater fame as an administrative revolution and a persecution — was the most successful of any reign in this compendium's second half: every war won, every frontier restored, no field disaster, and an abdication. The structure came first: within a year Diocletian had taken Maximian as colleague for the West (Augustus by 286), and in 293 completed the Tetrarchy with two Caesars, Constantius and Galerius — four emperors, four courts, four field armies, stationed not at Rome (which Diocletian visited once, late, and left early) but at the working capitals the strategy dictated: Nicomedia, Sirmium, Mediolanum, Trier. The system's military logic was the crisis's lesson: simultaneity answered with plurality, an emperor for every threatened front, usurpation pre-empted by making the probable usurpers colleagues. The reign's wars, compressed: on the Rhine and upper Danube, Maximian and Constantius broke Frankish and Alamannic incursions and settled the defeated inside the line as laeti; the Bagaudae — Gaul's peasant risings, the interior's first appearance in this series as a theater — were suppressed; Britain, seized in 286 by the fleet-commander Carausius (whose coinage, styling him a colleague of the emperors he defied, is the revolt's AGrade A voice) and held after him by his treasurer Allectus, was recovered in 296 by Constantius in a two-pronged Channel crossing through the fog, the event commemorated on the Arras medallion — the mounted emperor received at London's gate as REDDITOR LUCIS AETERNAE, restorer of the eternal light; Egypt's revolt under Domitius Domitianus was reduced by Diocletian in person with the eight-month siege of Alexandria (297), after which the southern frontier was drawn back to Philae and the Nobades subsidized to hold it; and in the East the reign fought the volume's first Persian war. Narses, Shapur's son, attacked Armenia in 296; Galerius, beaten in open country between Carrhae and Callinicum in 297 — the reign's one defeat, answered, the tradition says, by the emperor letting his purple-clad Caesar walk a mile before his carriage — rebuilt, struck through Armenia in 298, surprised the royal camp at Satala, captured the king's treasury, harem, and wives, and dictated the Peace of Nisibis of 299: Mesopotamia confirmed Roman with five trans-Tigris satrapies, Armenia enlarged under its Roman client, Nisibis the sole legal emporium — the most one-sidedly victorious eastern settlement in Roman history, and it held for forty years. Behind the wars, the machine: the provinces doubled in number and grouped under dioceses, civil and military career-paths severed, the army expanded (the fourth-century establishment's paper strength perhaps half a million) and re-equipped from state fabricae, the frontiers rebuilt in the squat, thick-walled, artillery-bearing fort-architecture whose remains from Britain's Saxon Shore to the Syrian strata Diocletiana are the reign's signature in stone; the whole funded by the new census and the annona's regularization — the crisis's emergency requisitions converted into the empire's permanent tax. On the first of May 305, in a ceremony on the hill outside Nicomedia where he had been acclaimed, Diocletian — first of all Roman emperors — abdicated, compelled his reluctant colleague to do the same, and retired to the palace-fortress at Split to grow, by the famous report, cabbages; the Editor marks the system's one great failure as the succession it was designed to solve: the Second Tetrarchy dissolved within eighteen months of its author's departure, because the sons the system had passed over — Maxentius in Rome, Constantine at York — declined to stay passed over, and the reorganized army proved as willing as the old one to be the instrument of its commanders' inheritance.

Constantine: The Civil Wars and the Christian Army306–337

The wars that ended the Tetrarchy occupied eighteen years and remade the world's religion, and the compendium narrates them as what they operationally were: the reunification of the empire by its best general. Constantine, acclaimed by his father's army at York in July 306, spent six years consolidating Gaul and Britain — Frankish kings thrown to the beasts at Trier, the Rhine bridged in stone at Cologne — while the system collapsed elsewhere (six Augusti at one point in 310, the count itself the verdict); in 312 he struck into Italy against Maxentius with a small, veteran field force, took Turin and Verona in the campaign's real fighting, and arrived before Rome, where Maxentius — abandoning the Aurelian walls that had defeated Galerius's and Severus's armies against every strategic logic, on the day of his accession-anniversary, with the Sibylline oracle promising the death of Rome's enemy — gave battle at the Milvian Bridge with the Tiber at his back and his bridge of boats behind him. The battle was brief: Constantine's cavalry broke the wings, the mass collapsed onto the pontoon crossing, the bridge failed, and Maxentius drowned in the press, his body recovered next day for its head. Above the campaign the tradition placed the vision — the cross of light over the sun, in hoc signo vinces, the dream, the Chi-Rho painted on the shields — transmitted in versions (Lactantius's dream, Eusebius's daylight sign sworn to him by the aged emperor decades later) that disagree in every particular except consequence; the Editor grades the vision as the compendium graded Sulla's dreams and the rain miracle — unrecoverable as event, AGrade A as the army's adopted fact — and notes what is materially certain: the Chi-Rho appears on Constantinian coinage and standards (the labarum) within the decade, the Arch of Constantine at Rome (315, built from spolia of Trajan, Hadrian, and Marcus — the visual history of Book VII dismantled to decorate its sequel) attributes the victory with studied vagueness to instinctu divinitatis, and the army of the West marched thereafter under a Christian monogram. The eastern reckoning came in two wars against Licinius — Cibalae and Campus Ardiensis in 316–317, the decade of armed truce, then the great campaign of 324: Adrianople stormed by feint and river-crossing in July, the Hellespont cleared by Crispus's fleet against Licinius's larger one, and the decision at Chrysopolis in September, after which the last colleague was interned at Thessalonica and, within the year, hanged. Constantine's sole reign (324–337) completed the military architecture this volume inherits: the field army formalized (comitatenses distinguished in law from ripenses by 325), the Guard abolished forever — the Praetorians, having backed Maxentius, were disbanded after the Milvian Bridge and their camp's walls slighted, closing an institution this compendium has watched kill emperors since Sejanus — and replaced by the scholae palatinae; the magistri militum created; Constantinople founded on the Bosporus (dedicated 330) as the strategic answer to the empire's actual center of gravity, equidistant from the Danube and the Euphrates; the Danube re-crossed in force (the Gothic war of 332, a client-treaty imposed on the Tervingi that held a generation; the stone bridge at Oescus; the Sarmatian interventions of 334) — and the reign ended in 337 with the emperor baptized on his deathbed at Nicomedia while the army he had rebuilt prepared the Persian war he did not live to fight, and his dynasty opened its succession, in the compendium's oldest pattern, with a massacre of its own collaterals.