Louis was allied with the Lombard principalities of southern Italy from the start, but an attempt at joint action with the Byzantine Empire failed in 869.

[6] The capitulary calls for more than an offensive action, it also ordered the construction of castles (castella) with palisades, outworks, moats and ramparts.

This probably required the use of the army, but for the next six months Louis peacefully toured the Lombard principalities of the Campania, assuring himself of their loyalty prior to his next move against Bari.

According to Erchempert, a contemporary witness, the princes of Benevento, Salerno and Capua all urged the emperor to attack Bari.

As part of these negotiations, a marriage may have been proposed between Louis's daughter, Ermengard, and Basil's eldest son, Constantine.

He had a claim to the city of Bari and also a strategic interest in the defeat of the emirate, which menaced Byzantine Dalmatia on the other side of the Adriatic Sea.

Louis argued in his letter that he had in fact already disbanded his main force for the winter because Niketas' fleet arrived so late in the year.

[1] Two Byzantine sources, On Administering the Empire and the Life of Basil the Emperor, refer to a Slavic contingent brought to the siege by the fleet of Ragusa.

"[2] According to Andrew of Bergamo, the people of Calabria sent envoys to Louis during the siege, offering allegiance and tribute in exchange for protection from the Saracens.

[19] Louis also mentions the arrival of enemy reinforcements from Sicily and Africa, apparently responding to the threat to Taranto, and he accuses Duke Sergius II of Naples of conspiring with the Aghlabids.

These troops did not move to retake Bari, however, but besieged Salerno in an effort to strengthen their position in Calabria and parts of the Italian peninsula nearer to Sicily.

Louis's continued presence in Benevento became an irritation to the Lombards, however, and on 13 August 871 he, his wife Engelberga and his daughter Ermengard were arrested by Prince Adelchis.

The Annals of Saint-Bertin record that Louis had been planning to send Adelchis into exile, and On Administering the Empire adds that this was a rumour spread by Sawdān.

[21] A contemporary poem, On the Captivity of the Emperor Louis, calls the imprisoned emir a "cunning assailant [or instigator]" (kalidus ille temtator).

According to the contemporary chronicler John the Deacon, Duke Sergius II of Naples and Prince Guaifer of Salerno had connived with Adelchis in Louis's imprisonment.