In the final years of his reign Constantine had widened the circle of the ruling dynasty. Two grandchildren of the collateral family line of Constantius and Theodora were promoted to high office in 335. Hannibalianus was made “king of kings and of the Pontic regions,” while his younger brother Dalmatius was appointed Caesar and placed in charge of Thrace, Macedonia, and Achaea. The political context of these new arrangements is far from clear, but in any case they were soon overtaken by the dynastic crisis which occurred at the death of Constantine on May 22, 337. None of the Caesars had been raised to the status of Augustus, and an interregnum extended until September 9. During this period Constantine's army, which Zosimus says would tolerate no emperor apart from Constantine's sons, is said to have taken matters into its own hands and murdered rival claimants to power, including Dalmatius, Hannibalianus, and all the other male descendants of Constantius and Theodora apart from Gallus and Julian, aged twelve and six years old respectively. Most sources, beginning with Julian himself, suggest that Constantius was the instigator of the murders.
The next phase of imperial history is poorly documented, until the surviving narrative of Ammianus Marcellinus picks up the story in 353. However, the main theme is continuing rivalry for power between Constantine's successors and other usurpers, resulting in a series of dynastic murders and civil wars. Constans, the ruler of the western empire, displaced his elder brother Constantine II from Africa and Italy, but was overthrown himself by Magnentius, a senior military commander of barbarian origin (from a community of laeti, German settlers, in northern Gaul), who executed a successful coup d'état at Autun on January 18, 350. Constantius, who had been dealing with sustained Sassanian aggression along the eastern frontier during the 340s, brought his troops into Europe to deal with the usurpation. His first major action was to neutralize the impact of Vetranio, another commander who had been proclaimed emperor by his troops in Pannonia in March 350. He then moved against Magnentius. Constantius secured his dynastic position by arranging for his cousin Gallus to marry his sister and raising him to the rank of Caesar. The first battle of the civil war was fought at Mursa on September 28, 351 (Map 3.2). The defeated Magnentius escaped to Gaul, where he joined his brother Decentius, whom he had appointed as his own Caesar. Their resistance was undermined by attacks from across the Rhine by the Alemanni, which had been subsidized by Constantius. The end came at the battle of Mons Seleuci, near Gap in the French Alps. Defeat led the rebels to take their own lives in August 353 (Zosimus 2.53).
Map 3.2 Illyricum. The battleground of the empire
Constantius was thus now sole ruler of the empire. His personality and ruling style are vividly depicted by Ammianus Marcellinus, whose detailed history begins for us with the events of 353. Constantius is represented as being harsh and irritable in temper, and notably prone to baseless suspicion of would-be conspirators and rivals, a tendency which was exacerbated by the flattery of his courtiers. The most prominent victim of Constantius' political insecurity was the Caesar Gallus, who was suspected of treasonous behavior in Antioch, summoned to the West, and executed at Histria in 354 (Ammianus 14.11.20–28). As the historian puts it, Constantius' courtiers encouraged the belief that the fate of the entire world hung on the slender thread of the emperor's own life. This sentiment correlated precisely with the tone of panegyric oratory in the fourth century, which relentlessly propounded the view that the empire's well-being in every respect depended on the vigilance, providence, and courage of the emperors.
The drastic sequence of usurpation, civil war, and dynastic murder which characterized the empire during the 350s led to wider insecurity. Zosimus accurately summarizes the situation at the beginning of his third book:
Everywhere (Constantius) saw the Roman empire being dismembered by barbarian incursions: the Franks, Alemanni and Saxons had already taken forty cities on the Rhine and left them in ruins by carrying off countless numbers of their inhabitants and untold spoils; the Quadi and Sarmatians had very boldly overrun Pannonia and Upper Moesia; and the Persians were continually harassing the East, whereas previously they had been inactive for fear of being attacked by Gallus Caesar. (Zosimus 3.1, trans. Ridley)
Gaul in particular had been devastated first by the war requisitions of Magnentius and then by the consequences of his defeat. The Alemanni were incited to raid and devastate the country, and the appointment of Silvanus to deal with them had ended in a self-inflicted catastrophe. It was against this background that Constantius appointed a new Caesar, Gallus' younger half-brother Julian, who was summoned from his studies at Athens to the court at Milan in 355 to undertake imperial responsibilities. Julian's cause was championed by Constantius' wife, the beautiful but childless Eusebia, and his position at the heart of the ruling family was strengthened by marriage to the emperor's sister Helena.
During the late 350s the empire was exposed to more external pressure than at any time since the 290s. Julian campaigned with unexpected energy and success against Frankish and Alemannic incursions in Gaul throughout this period. In 355 and 356 Constantius, based at Milan, also mounted expeditions against Alemannic tribes in the upper Rhine valley. In 357 he paid a famous visit to Rome, but returned to the Balkan frontier headquarters at Sirmium in 358 and 359, in order to deal with threats to the middle Danube area from the Sarmatian Limigantes. During the emperor's long absence in the West, the Sassanian king Sapor II increased pressure on the Mesopotamian front, capturing the city of Amida (Diyarbakır) in 359. Constantius returned to the East, spent the winter of 359–60 in Constantinople, and in the following spring marched as far as Cappadocia, before he heard news that his Caesar Julian had been proclaimed Augustus by his troops in Paris. Even so, the Sassanian threat was too serious to ignore (the frontier cities of Bezabde and Singara were taken in 360), and he continued to make a show of strength in Syria and across the Euphrates in Mesopotamia over two campaigning seasons, before finally turning back from Antioch to face the threat of civil war in autumn 361. A confrontation with Julian loomed, but Constantius died in Cilicia at the road station of Mopsucrene, in the foothills of the Taurus mountains, on November 3, aged forty-four.