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; Comiaculum ed. Westerbergh.
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 July 4, 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. 111-115). 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 sanctę nostrę ecclesię invaserunt hereditatem, insuper et fidem pastoralem rectitudinis in ipsa Histriensi provincia abdicarunt ") - edited here as doc. 768_IS.
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.
The fact 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.
Bernardo Benussi (cited above), claimed that King Aistulf conquered Istria in 751, as narrated in the Salerno Chronicle, and that the Lombards held the region until the Frankish conquest of 774. Patriarch John's letter would thus simply present an episode taking place during the Lombard dominion over Istria, one where the Lombards began to increasingly intervene in ecclesiastical matters. Pirchegger (cited above), follows Benussi.
Walter Lenel (cited above) disagreed and claimed that the Lombard dominion over Istria began no sooner than 770, with the conquest of King Desiderius. He thus completely ignored the Salerno Chronicle as a historical source.
Erich Caspar (cited above) followed Lenel and built upon his thesis. The Lombards did attack Istria in 751, but failed to conquer the region. This interpretation would be corroborated by the fact that neither of the two treaties of Pavia (755/756) mention Istria (discussed in doc. 754_PC). Like Lenel, Caspar dated the beginning of the Lombard occupation of Istria to the age of King Desiderius, in a period between 768/772, according to Patriarch John's letter to Pope Stephen III.
According to Giovanni De Vergottini (cited above), the Lombards conquered Istria twice, but did not manage to hold it for long: first in 751 when they lost it almost immediately afterward; 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. To this day, this is the most widely accepted interpretation of these events.
Pio Paschini (cited above) did not believe in any Lombard occupation of Istria, only in two attacks; the region, argued Paschini, remained in Byzantine hands until the Frankish takeover of "c. 790" (recte 788).
Roberto Cessi (cited above) 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. This thesis is nowadays largely refuted (cf. the Editori's Comments in this document).
Federico Seneca (cited above) followed Cessi, whereas Lujo Margetić (cited above) also voiced his criticism towards the Salerno Chronicle, dubbing it "an unreliable source".
Thomas F. X. Noble (cited above) argued that King Aistulf might have attacked Istria, but that he failed to conquer it, dating the Lombard conquest of the region to the age of King Desiderius, pretty much following Caspar's interpretation.
Rajko Bratož (cited above), is less skeptical towards the Salerno Chronicle, delineating the "Lombard phase" of Istrian history very much like Benussi, from 751 to 774.
More recently, Maurizio Levak is also skeptical 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.
Egidio Ivetic (cited above), follows the traditional interpretation, claiming that Istria was occupied by the Lombards in 751, as narrated in the Salerno Chronicle.
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. Patriarch John’s letter to Pope Stephen III can be read in several ways and is in no way undisputed evidence of the Lombard’s “recent” conquest of Istria, as Cessi forcefully reads it (cf. the editor’s comments in doc. 768_IS). Therefore, the chronology established by Bernardo Benussi and Hans Pirchegger - Istria under Byzantium from c. 539 to 751, from 751 to c. 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, perhaps even intermittent, devoid of any deeper structural modifications of the existing Byzantine administrative framework.
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