the concerns of this chapter may usefully be introduced by consideration of three episodes, each, as it happens, falling within a period of less than a decade during the middle of the fourth century and each recounted by the historian Ammianus Marcellinus.

The first concerns the activities during the mid-350s of the praetorian prefect of the east, Strategius Musonianus. He is reported to have investigated Persian plans through the agency of spies (speculatores), from whom he and an associate learned (aperte cognossent) that the Persian king Shapur II was currently engaged in fierce fighting with hostile peoples on a distant frontier of his empire. On the basis of this information Musonianus initiated secret negotiations with a Persian official, in the hope that these difficulties would incline the Persians towards a formal peace settlement with the Romans and put an end to the costly but inconclusive warfare of the previous two decades (xvi.9.2-3).

In the event, Musonianus' diplomatic initiative proved not only abortive, but positively counter-productive, leading on to the second episode. The Persians concluded from Musonianus' initiative that the Romans themselves were in difficulties, and since Shapur had been able, by early 358, to bring his own war to an end, he decided in turn to try to exploit Roman problems and sent envoys to demand territorial concessions as the price for peace.