The Albanians, who were already present in the strategically important triangle between Durrës, Ohrid and Corfu during the power struggles of the 13th century, were in great demand as allies and mercenaries; they increasingly expanded outside of their mountainous homes and began imposing themselves in the surrounding lowlands.
[2] At the beginning of the 14th century, Albanian tribes began migrating to Thessaly, where they lived primarily as autonomous nomads who controlled the mountainous parts of the region.
[4] After 1318, the Albanian tribes began to invade and ravage the countryside of Thessaly instead of continuing their peaceful migrations, forcing the Greek and Catalan authorities to withdraw to their strongholds.
[4] Prior to the outbreak of the civil war, Byzantine general Cantacuzene had planned to lead an army westwards to subdue the Albanians of Pogoniani and Livisda, who were regularly raiding and plundering towns as far south as Akarnania and Balagrita.
Due to the support he received from his brother-in-law and governor of Epirus Symeon Uroš as well as locals who were both happy to rid themselves of Serbian rule and eager for help against the Albanians, Nikephoros conquered Thessaly without much difficulty.
Despite their reunion, Nikephoros, who was advised to seek a political marriage, left Maria under guard in Arta and proposed to a Serbian princess who was the sister of Stefan Dušan's widow, Helena of Bulgaria.
After escaping to the Morea with her brother Manuel Cantacuzene's help, Maria's plight and Nikephoros' pursuit of a Serbian alliance angered the Albanians, leading to threats of war against him.
[4][2][10][1][6] The exact date of the Battle of Achelous has been debated, with primary sources suggesting it occurred three years, two months, and a few days after Nikephoros' arrival in Thessaly in 1356, placing his death in 1359.
However, discrepancies in historical accounts and theories by scholars like J. Voyatzidis and C. Hopf suggest alternative dates ranging from 1358 to 1359, based on interpretations of conquest timelines and the involvement of key figures.
Ultimately, Hopf's interpretation, aligning with most of the evidence available, suggests the battle occurred in late spring or early summer of 1358, despite debates over the exact year.
Attributions to a later year and even a different leader for the Albanians (claimed by some to be Gjin Bua Shpata and Peter Losha rather than Karl Thopia) are probably based on misunderstandings of primary sources that were referring to different events.
Symeon then focused on reclaiming his former holdings in Epirus, and since most of the major towns in the region were threatened by the Albanian tribesmen, they quickly submitted to his rule.
[6] The governors Symeon appointed prior to his departure from Epirus failed to control the region, and the migration of the Albanians continued in ever greater numbers.
[6] The city remained under Greek governance, and as a traditional centre of Byzantine imperial ideology, Ioannina made every effort to resist the threat of the Albanians.