The Battle of Halmyros, (known also as; the Battle of the Cephissus or Battle of Orchomenos), was fought on this day in 1311 between the forces of the Frankish Duchy of Athens and its vassals under Walter of Brienne and the mercenaries of the Catalan Company, resulting in a devastating victory for the Catalans.
Engaged in conflict with their original employers, the Byzantine Empire, the Catalan Company had traversed the southern Balkans and arrived in southern Greece in 1309. The new Duke of Athens, Walter of Brienne, hired them to attack the Greek ruler of neighbouring Thessaly. Although the Catalans conquered much of the region for him, Walter refused to pay them the salaries owed, and prepared to forcibly expel them from their gains. The two armies met at Halmyros in southern Thessaly (or at the Boeotic Cephissus, near Orchomenos, according to an earlier interpretation). The Catalans were considerably outnumbered and weakened by the reluctance of their Turkish auxiliaries to fight. The Company did have the advantage of selecting the battleground, positioning themselves behind marshy terrain, which they further inundated with water. On the Athenian side, many of the most important lords of Frankish Greece were present and Walter, a prideful man and confident in the prowess of his heavy cavalry, proceeded to charge headlong against the Catalan line. The marsh impeded the Frankish attack and the Catalan infantry stood firm. The Turks, seeing that battle was joined in earnest, re-joined the Company, and the Frankish army was routed; Walter and almost the entire knighthood of his realm fell in the field. As a result of the battle, the leaderless Duchy of Athens was taken over by the Catalans, who ruled that part of Greece until the 1380s.