Yikes the hiatus was longer than expected! Hopefully I shall do better in the future. I am also nixing the fake book method and this will be more a straight narrative TL with copious fake primary sources. Anyways, here's something new!
Initially, the war seemed to go in Charles’ favour. An inopportune spring snowstorm in the Alps delayed Desiderius’s Lombards from aiding Carloman’s troops. Moreover, while Charles’ lands were more susceptible to attack, they were far richer and more integrated than his brother’s central realms, allowing Charles an advantage in taking the field earlier with a better equipped and integrated army. The troops yet remaining in western Aquitaine from Hunald’s rebellion just three years before and his fierce Austrasian veterans of Saxon skirmishes allowed Charles to take many of his brother’s bordering possessions. By the winter of 772, things were looking grim for Carloman, as his brother slowly but surely pushed the borders east and south, even taking his capital at Soissons, forcing the relocation of his court to Arvernis[1], and the main forces of the Lombards still remained south of the Alps, unable to come to his aid. Moreover, Duke Nebi of Alammania, grandfather of Hildegard, Charles’ young new queen, rebelled against Carloman’s overlordship and threw his support behind the banner of his brother, sending troops under his son Gerold to battle against Carloman’s eastern garrisons. It was only in the far-eastern holdings of Charles that Carloman’s alliance was making headway under Duke Tassilo’s Bavarians. Indeed, as Abbot Fulrad of Saint Denis recounted in his letters, “Charles, headstrong and cocksure, thought to end this war within a few months and brought almost all his troops to bear against his brother, leaving only a few garrisons to guard the northern borders.” Eager eyes in the north perceived this development with glee, with several initial probing raids by the Saxons lead by the dynamic young Widukind testing the depleted forts in the north by the spring of 773.
While brazen Charles won by the sword, Carloman and his allies’ agents would work wonders with the pen. Under the urging of his chief advisor, Autcher, Carloman sent emissaries across the Pyrenees to the court of Lupo II of Vasconia, asking for his aid. Lupo, wary of Charles’ seemingly likely success, begged off for the time being. Carloman also sent a representative to the restive elements in Charles’ Aquitaine, the simmering distaste for Frankish rule from the revolts three years prior fanned to a flame by the high taxes enacted by Charles to pay for the war. The rebellious elements coalesced around a charismatic figure named Felix, who asserted to be the son of Hunald, the slain leader of the most recent revolt. Delayed by the storm, Desiderius did not let the time go to waste. He began to restore the now-dilapidated fortifications of his capital Pavia and ordered the roads of his realm to be repaired to allow for faster army movement. It was at this time, or so the Lombard chronicler and secretary to Desiderius, Paul the Deacon wrote in his ten-volume Historia gentis Langobardum [2], that Desiderius had
“a prophetic dream wherein St. Thomas the Apostle appeared unto him and pointed east to a vast plain, saying “King, your cause is just but you are too few! Do as I did and look to the east! Send the scorned fruit of your seed and seal not only victory in the world but in the celestial kingdom! Let those who were once the greatest enemies of Christendom be made its most fervent defenders.’ Having woken, the king immediately sprang to his feet, summoning his daughter Gerperga to him and sending her, with the blessing of the Pope, along with an embassy of noblemen and a group of papal missionaries lead by the esteemed clergyman Hadrian [3], to court of King Kaganus[4] of the Avars of Pannonia.”
[1] OTL Clermont-Ferrand.
[2] Paul has a chance to finish his history, which goes up to the end of Desiderius’ reign instead of just Luitprand’s.
[3] OTL’s Pope Adrian I. Using Paulus Afiarta, the leader of the Lombard party in Rome, Desiderius convinced Pope Stephen to sent Adrian on the mission, thus removing the most charismatic and vocal anti-Lombard from Italy.
[4] Paul here is misunderstanding the title of the Avars’ leaders, khagan, as the given name of a king (the fact that I cannot find any record of who ruled the Avars until between 630 and 790 has nothing to do with it, no sir!).