The Achaeans launched a surprise attack on the greatly superior and overconfident Byzantine force, defeated and scattered it, saving the principality from conquest.

At the Battle of Pelagonia (1259), the forces of the Byzantine emperor Michael VIII Palaiologos (r. 1259–1282) killed or captured most of the Latin nobles of the Principality of Achaea, including the Prince William II of Villehardouin (r. 1246–1278).

[1][2][3] The agreement was bound to be of short duration, however: the establishment of a small province in the Morea was for Palaiologos but the first step towards reclaiming all of the peninsula, and William likewise was involved in the Latin efforts to counter the emperor and regain Constantinople.

In July, Pope Urban IV nullified William's oaths to the emperor, and appealed to the Western princes for aid against the "schismatic" Byzantines.

[7] Written by and for the Frankish nobility of the Morea, as a source it is very biased against the Byzantines, who are stereotypically "depicted as effeminate, devious and cowardly", while the Franks are portrayed as "virtuous and almost suicidal in their courage".

[15] The Chronicle does not report on these events apart from the siege of Lacedaemon, but Pachymeres, Nikephoros Gregoras, and Sanudo record that "daily clashes" took place with the Franks, and that several strongholds fell to the imperial troops; these are not named, but must have included the castles of Passavant, Geraki, and Beaufort (Leuktron).

Abandoning the fruitless siege of Lacedaemon, he marched his army up the rivers Eurotas and Alfeios towards the Achaean capital, Andravida, on the northwestern coast of the Morea.

Although the general outline of the subsequent events is confirmed from the report of the Venetian historian Marino Sanudo, the only detailed account available is the narrative of the Chronicle of the Morea, whose accuracy has been questioned.