Lombard King Aistulf defeats and captures Eutychius, the Exarch of Ravenna, conquering the Byzantine Exarchate in Italy and further expanding the Lombard Kingdom over Comacchio, Ferrara and Istria (narrative account from the Salerno Chronicle).
Post hunc1 in Regnum est elevatus Aystulfus, vir per omnia astutissimus et ferox.
Per idem tempus Euthicius Romanorum patricius se Aystulfo tradidit, simulque [Com]iaculuma atque Ferrariam seu et Istriam pugnando optinuit.
a) iaculum B.
1) Postquam Ratehis regalem dignitatem reliquerat (anno Domini 749).
Afterwards, Aistulf, a most cunning and fierce man, was elevated [to king] in the Kingdom.
During this time, Eutychius, a Roman patrician, surrendered himself to Aistulf who simultaneously conquered in battle Comacchio, Ferrara and Istria.
This is the only primary source explicitly attesting to the Lombard conquest of Istria. However, the Chronicon Salernitanum is a narrative source penned centuries later, during the final decades of the 10th century. Nonetheless, the statement that Aistulf conquered the Exarchate of Ravenna is indeed corroborated by other primary sources: namely the privilege to the monastery of St. Mary in Farfa that King Aistulf proudly issued from the palace in Ravenna on the 4th of July, 751 (the document is best edited in Carlrichard Brühl (ed.), Codice diplomatico longobardo, vol. 3/1, Fonti per la storia d'Italia 64 (Rome 1973), doc. 23, pp. 111-15). Therefore, the conquest of Ravenna is dated to 751 and since the Chronicle speaks of the simultaneous conquest of Comacchio, Ferrara, and Istria, these events are also dated to 751.
There are, however, problems with such an early dating of the Lombard conquest of Istria. Namely, there are no contemporary primary sources from the Northern Adriatic region that would confirm the Lombard takeover of Istria already in the 750s. The oldest document attesting to Lombard jurisdictions in Istria is dated between 768 and 772: a letter issued by Patriarch John of Grado to Pope Stephen III referring to the Lombard conquest of Istrian Churches ("gens perfida Langobardorum sanctae nostrae Ecclesiae invaserunt hereditatem, insuper et fidem pastoralem rectitudinis in ipsa Histriensi provincia abdicarunt") - see the source here.
Moreover, the acts of the famous Placitum Rizianense of 804 refer multiple times to the Byzantine administration in Istria, but there is absolutely no mention of any Lombard jurisdictions - see the source here.
These two facts, that besides the Salerno Chronicle the earliest mention of Lombard invasion of Istria stems from the age of King Desiderius (757-774) and that this Lombard rule was so effete that it was not even mentioned in the Plea of Rižana, engendered several historiographic interpretations of the Lombard conquest of Istria.
According to Giovanni De Vergottini, the Lombards conquered Istria twice, but did not manage to hold it for long: first in 751 when they lost it almost immediately afterwards; and then in a period between 768 and 772 under King Desiderius with their effete rule lasting only until 774, with little or no consequences in the administrative framework of the region (De Vergottini, Lineamenti storici della costituzione politica dell'Istria durante il Medio Evo, 2nd ed. (Trieste 1974), pp. 18-19). To this day, this is the most widely accepted interpretation of these events.
Roberto Cessi also based his argumentation on the correspondence between Patriarch John of Grado and Pope Stephen III when he concluded that the Lombard conquest of Istria happened c. 770 and lasted uninterrupted until the Frankish takeover of 774 (Cessi, "L'occupazione," 290-97). This thesis is nowadays largely refuted (cf. this document).
Most recently, Maurizio Levak is also sceptical of the Lombard conquest of Istria as narrated by the Salerno Chronicle. While there is a possibility that the Lombards ruled briefly over (parts of) Istria between 770 and 774, argues Levak, their administration was largely inconsequential (Levak, "Istra i Kvarner u ranome srednjem vijeku," in Nova zraka u europskom svjetlu: Hrvatske zemlje u ranome srednjem vijeku (oko 550 – oko 1150), ed. Zrinka Nikolić Jakus (Zagreb 2015), p. 396).
In the end, it must be concluded that the surviving primary sources, few as they are, indeed speak in favor of the thesis that the Lombard rule in Istria was brief, effete, and with little to no consequences to the jurisdictional framework of the region. Nonetheless, there are absolutely no primary sources that would attest to King Desidarius' conquest of Istria. Therefore, the chronology established by Bernardo Benussi and Hans Pirchegger - Istria under Byzantium from c. 539 to 751, from 751 to 774 under the Lombards, from 774 to 788 under Byzantium again, and from 788 under the Carolingians - is followed here as well, albeit with an addendum that Lombard administration of the peninsula was effete and provisional, devoid of any deeper structural modifications of the existing Byzantine administrative framework.
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