The Battle of Halmyros, known by earlier scholars as the Battle of the Cephissus or Battle of Orchomenos, was fought on 15 March 1311, between the forces of the Frankish Duchy of Athens and its vassals under Walter of Brienne against the mercenaries of the Catalan Company, resulting in a decisive victory for the mercenaries. 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 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. 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 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 Catalans took over the leaderless Duchy of Athens; they ruled that part of Greece until the 1380s. Background Following the Sack of Constantinople in 1204, much of Greece came under the rule of Frankish Crusader principalities. The most notable of them were the Kingdom of Thessalonica, the Principality of Achaea, and the Duchy of Athens, with its capital at Thebes. Thessalonica proved short-lived and fell to the resurgent Greeks, but the other Frankish principalities persevered and even prospered for most of the 13th century. In his landmark 1908 history of Frankish Greece, the medievalist William Miller writes of the Duchy of Athens that "under the dominion of the dukes of the house of de la Roche, trade prospered, manufactures flourished, and the splendours of the Theban court impressed foreigners accustomed to the pomps and pageants of much greater states." On 5 October