Emperor Justinian I appoints his cousin Germanus as the commander-in-chief of his army fighting against King Totila in Italy; the promise of a renewed war effort against the Ostrogoths under a famous military commander encourages men across the region to rally under imperial banner as they journey to Istria, awaiting the arrival of Germanus (narrative accounts from Procopius of Cesearea's History of the Wars).
αὐτοκράτορα δε τοῦ [πρὸς Τουτίλαν τε και Γότθους]a πολέμον Γερμανὸν κατεστήσατο τὸυ αὑτοῦ ἀνεψιόν. ὧ δὴ στράτευμα μὲν οὐ πολὺ ἔδωκε, χρήματα δὲ λόγου ἄζια παροσχόμενος στρατιὰν ἐπέστελλεν ἔκ τε θρᾳκῶν καὶ Ίλλυριῶν ἀζιολγωτάτην ἀγείρανταb οὔτω δὴ στέλλεσθαι σπουδῇ ἐς τὴν Ίταλιαν πολλῇ.
οἷς δὴ ἅπασι θαρσήσαντες οἱ τοῦ βασιλέως στρατοῦ ἔν τε ʹΡαβέννῃ καὶ εἴ που ἄλλη πόλις σφίσι λελεῖφθαι τετύχηκεν, εὐέλπιδες ἰσχυρότατα γεγενημένοι, τά χωρία βασιλεῖ ἐς τὸ ἀκριβὲς φυλάσσειν ἠζίουν.
ἀλλὰ καὶ ὄσοι ζὺν τῷ Βήρῳ τὰ πρότερα ἢ ἄλλοις τισὶ [τοῖς]c πολεμίοις ἐς χεῖρας ἐλθόντες ἡσσημένοι τε τῶν ἐναντίων ἐν τῇ ξυμβολῇ διέφυγόν τε καὶ σκεδαννύμενοι περιῄεσαν, ὅπη ἑκάστῳ τετύχηκεν, ἐπειδὴ ὁδῷ ἰέναι Γερμανὸν ἤκευσαν, ἀθρόοι ἐν Ίστριᾳ γεγενημένοι, ἐνταῦθά τε τὸ στάτευμα τοῦτο προσδεχόμενοι, ἡσουχῆ ἔμενον.
a) om. B, cf. ed. Haury. b) ἀγείραντι ed. Haury. c) om. B, cf. ed. Haury.
But as commander-in-chief in the war against Totila and the Goths he appointed Germanus, his own nephew. To him he gave an army of no great size, but he provided him with a considerable amount of money and directed him to gather a very formidable army from Thrace and lllyricum and then to set forth with great speed for Italy.
All these things brought fresh courage to the detachments of the emperor's army in Ravenna and whatever other cities chanced to be left in their hands, and being now filled with the highest hopes they were determined to guard the towns rigorously for the emperor.
Nay, more, all those who under Verus or other creams had previously engaged with the enemy and had escaped after being defeated in battle by their opponents, and were now dispersed and wandering about, each man wherever chance led him, all these, as soon as they heard that Germanus was on the way, gathered in a body in Istria, and there remained quiet, awaiting this army.
[translation taken from Procopius, History of the Wars, trans. Henry B. Dewing, vol. 5 (London 1962), pp. 31, 35]
The final explicit mention of Istria in the History of Wars penned by 6th-century Byzantine historian Procopius of Caesarea. Similarly to two mentions of the region that preceded it (see the sources here and here), there are various historiographic interpretations.
The communis opinio is that Istria was at this time, in 550, under the (Eastern) Roman Empire, conquered in the first phase of the Gothic War in 539.
However, since Germanus died in the same year he had been made the commander-in-chief and thus never managed to reach Istria, Antoljak argued that it cannot be inferred that the Peninsula remained under Byzantine authority. For Antoljak, Pula was the only part of Istria outside of Ostrogothic authority and even this city was (re)conquered by Totila's Ostrogoths in 550. The Peninsula was thus only subsequently conquered from Ostrogoths by the imperial army led by Narses in 552.
The period of Gothic rule in Istria mentioned by Pope Pelagius I in his letter to Patrician Valerianus is the only primary source that would seemingly support Antoljak's thesis, but the chronology of this period of putative Ostrogothic rule cannot be more precisely defined (see the source here).
Taking all the passages from Procopius' History of the Wars that mention Istria together, the most logical interpretation ends up being the one put forth and elaborated by Bernardo Benussi back in 1897: that Ostrogothic Istria was indeed conquered in 539 by the imperial army of Vitalius as he journeyed from Salona to Ravenna to reach Belisarius (see the source here). While it remains a possibility that Totila managed to retake some (continental) parts of the Peninsula sometime in the (late) 540s, this Ostrogothic rule in Istria can only be characterized as provisional and effete.
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