In the Battle of Halmyros, the Catalan Company defeated the forces of Walter V, Count of Brienne, taking control of the Duchy of Athens, a Crusader state in present-day Greece.

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.

The Catalan Company or the Great Catalan Company was a company of mercenaries led by Roger de Flor in the early 14th century and hired by Byzantine Emperor Andronikos II Palaiologos to combat the increasing power of the Anatolian beyliks. It was formed by almogavar veterans of the War of the Sicilian Vespers, who had remained unemployed after the signing in 1302 of the Peace of Caltabellotta between the Crown of Aragon and the French dynasty of the Angevins.

Walter V of Brienne was Duke of Athens from 1308 until his death. Being the only son of Hugh of Brienne and Isabella de la Roche, Walter was the heir to large estates in France, the Kingdom of Naples, and the Peloponnese. He was held in custody in the Sicilian castle of Augusta between 1287 and 1296 or 1297 to secure the payment of his father's ransom to the Aragonese admiral Roger of Lauria. When his father died fighting against Lauria in 1296, Walter inherited the County of Brienne in France, and the counties of Lecce and Conversano in southern Italy. He was released, but he was captured during a Neapolitan invasion of Sicily in 1299. His second captivity lasted until the Treaty of Caltabellotta in 1302.

The Duchy of Athens was one of the Crusader states set up in Greece after the conquest of the Byzantine Empire during the Fourth Crusade as part of the process known as Frankokratia, encompassing the regions of Attica and Boeotia, and surviving until its conquest by the Ottoman Empire in the 15th century.