A job for Gallus
Constantius II was well aware that running the empire was no longer a job for one man. He now remembered that he had two surviving cousins, namely Gallus and Julian, still languishing in exile ten years after he had placed them there. He gave Gallus the rank of Caesar together with the task of keeping order in the east while Constantius was elsewhere. Julian was also released and was allowed to get himself an education at Constantinople.
The armies of Constantius and Magnentius met in battle at Mursa (in modern Croatia) in 351. It was a victory for Constantius but both sides suffered huge casualties. After a second battle in 353, at Mons Seleucus in Gaul, Magnentius committed suicide.
Meanwhile, Gallus made the mistake of exercising more power than he had been authorized to do. Constantius was not a man to take that sort of thing lying down and he had his cousin executed for treason. The same fate befell another pretender in the west, named Silvanus. Constantius now called on Gallus’s brother, Julian, to be his Caesar in the west while he returned to take full control in the east.
A job for Julian
Julian was an interesting character, not least because he threatened to undo the move of the empire towards Christianity and return it to paganism. He proved to be a wily operator, which was a good quality to have when his boss was a tyrant like Constantius who had already executed just about everyone in sight who was related to him, leaving Julian as the sole survivor.
Julian reckoned that the way to make himself look good in Constantius’s eyes was to arrange matters so that all failures in the western empire could be ascribed to other people. This was what he did when a band of raiders, the Alamanni, invaded his territory and Julian left his commanders to take the blame for the failure to repel them. Almost by accident, Julian managed to capture the king of the Alamanni, which duly impressed Constantius who then began to entrust Julian with real authority.
Julian also copied Constantius by only dealing with officials at one remove – via correspondence and emissaries – thus instilling a sense of fear in them. This ensured that nobody stepped out of line.
However, Julian was also ambitious and he felt that he was destined to become emperor. Constantius may have suspected that Julian could not be entirely trusted, but he had problems of his own to deal with on his eastern front thanks to Shapur II once again threatening to invade.
In the summer of 359 the Persians invaded and besieged the city of Amida (modern Diyarbakır in eastern Turkey). The city fell eventually, but the Persians then withdrew because of the time they had lost in mounting a summer campaign. Constantius knew that they would return, but in order to defeat them he needed more troops, which could only come from the west and be supplied by Julian.
This gave Julian the opportunity he needed to exploit the widespread discontent felt in the west towards Constantius. In March 360 he allowed himself to be declared Augustus and to propose to Constantius that the empire should be split between them, which was a deal that Constantius had no choice but to accept.
In 361 Julian went even further by leading an army into the Balkans, this being an obvious threat to Constantius’s authority. However, before open warfare could erupt Constantius died of a fever, leaving Julian in sole command of the empire.