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). On the Athenian side, many of the most important lords of Frankish Greece were present. The Catalans were considerably outnumbered and weakened by the reluctance of their Turkish auxiliaries to fight. They did have the advantage of selecting the battleground, positioning themselves behind marshy terrain, which they further inundated. Walter, a prideful man and confident in the prowess of his heavy cavalry, charged 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, with Walter and almost the entire knighthood of his realm falling in the field. Subsequently the Catalans took over the leaderless Duchy of Athens, ruling that part of Greece until the 1380s.
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 1308, the last de la Roche Duke of Athens, Guy II, died childless. His succession was disputed, but in mid-1309, the High Court (feudal council) of Achaea chose his cousin, the Burgundian noble Walter of Brienne, as successor.
At that time the Greek world was in turmoil owing to the actions of the Catalan Company, a group of mercenaries, veterans of the War of the Sicilian Vespers, originally hired by the Byzantine Empire against the Turks in Asia Minor. Mutual suspicion and quarrels led to war with the Byzantines; evicted from their base in Gallipoli in 1307, the Catalans fought and pillaged their way west through Thrace and Macedonia, until, pressed by Byzantine troops under Chandrenos, they entered Thessaly in early 1309. The last leader of the company, Bernat de Rocafort, had envisaged the restoration of the Kingdom of Thessalonica with himself at its head, and had even entered into negotiations for a marriage alliance with Guy II. Nothing came of these negotiations, as Rocafort's increasingly despotic rule led to his deposition. After that, the company was ruled by a committee of four, assisted by a twelve-member council. The arrival of the company's 8,000 men in Thessaly caused concern to its Greek ruler, John II Doukas. Having just exploited the death of Guy II to repudiate the overlordship of the Dukes of Athens, John turned to Byzantium and the other Greek principality, the Despotate of Epirus, for aid. Defeated by the Greeks, the Catalans agreed to pass peacefully through Thessaly towards the Frankish principalities of southern Greece.
Walter of Brienne had fought the Catalans in Italy during the War of the Vespers, spoke their language, and had gained their respect. Using this familiarity, he now hired the company for six months against the Greeks, at the high price of four ounces of gold for every heavy cavalryman, two for every light cavalryman, and one for every infantryman, to be paid every month, with two months' payment in advance. Turning back, the Catalans captured the town of Domokos and some thirty other fortresses, and plundered the rich plain of Thessaly, forcing the Greek states to come to terms with Walter. This brought Walter accolades and financial rewards from Pope Clement V, but the Duke now declined to honour his bargain with the Catalans and provide the remaining four months' pay. Walter picked the best 200 horsemen and 300 Almogavar infantry from the company, paid them their arrears and gave them land so they would remain in his service, while ordering the rest to hand over their conquests and depart. The Catalans offered to recognize him as their lord if they were allowed to keep some of the land they had taken to establish themselves but Walter rejected their proposal and prepared to expel them by force. The Duke of Athens assembled a large army, comprising his feudatories—among the most prominent were Albert Pallavicini, Margrave of Bodonitsa, Thomas III d'Autremencourt, Lord of Salona and Marshal of Achaea, and the barons of Euboea, Boniface of Verona, George I Ghisi, and John of Maisy—as well as reinforcements sent from the other principalities of Frankish Greece.
Sources and location of the battle
A number of sources report in various degree of detail on the events before and during the battle: chapter 240 of the chronicle of Ramon Muntaner; the various versions of the Chronicle of the Morea (sections 540 and 548 of the French version, verses 7263–7300 and 8010 of the Greek version, and sections 546–555 of the Aragonese version), Book VII section 7 of the history of the Byzantine writer Nikephoros Gregoras, and brief accounts in Book VIII of the Nuova Cronica of the Florentine banker and diplomat Giovanni Villani, in the Istoria di Romania of the Venetian statesman Marino Sanudo and in letter of the latter that remained unpublished until 1940.
The location of the battle varies in the different sources between two locations. Muntaner reports that it took place "at a beautiful plain near Thebes, where there were marshes", which has been identified with the plain of the Boeotic Cephissus and the marshes of Lake Copais (now drained). Gregoras likewise mentions that the battle took place near the Boeotic Cephissus. On the other hand, the versions of the Chronicle of the Morea place the battle at "Halmyros", apparently the town of the same name in southern Thessaly, where there was another city known as Thebes. The former localization had been long favoured in scholarship; in his standard history of Frankish Greece, William Miller rejected Halmyros on the basis of the topography described by Muntaner, a view that continues to be repeated in more recent works. Several proposals by modern scholars for the exact site of the battle in the Cephissus valley exist, ranging from the vicinity of Orchomenos and the vicinity of Copais to locations further north, around the villages of Chaeronea and Davleia, or even Amfikleia and Lilaia.
The critical examination of the primary sources by more recent scholars has reversed the situation. Muntaner was himself a member of the company until 1307, but was posted as governor of Djerba when the battle occurred and only compiled his chronicle in 1325–1328, leading to some serious errors in his account. Gregoras, although a contemporary of the battle, wrote his history even later, in 1349–1351, relying mostly on second-hand sources; his understanding of the company's activities during the years before the battle is sketchy and inaccurate, and his account of the battle itself is very close to that of Muntaner, indicating perhaps that Gregoras drew on a Western source. On the other hand, the original French version of the Chronicle of the Morea, on which all other versions draw, was written between 1292 and 1320, and the abridged French version surviving today was compiled shortly after by a well-informed author in the Morea. The Greek and Aragonese versions, compiled later in the century, contain essentially the same information as the French version. A critical piece of evidence was the discovery and publication in 1940 of a 1327 letter by Marino Sanudo, who was a galley captain operating in the North Euboean Gulf on the day of the battle. Sanudo clearly states that the battle took place at Halmyros ("... fuit bellum ducis Athenarum et comitis Brennensis cum compangna predicta ad Almiro"), and his testimony is generally considered reliable. As a result, more recent historical studies commonly accept Halmyros as the site of the battle.