Theodore died in 1303, leaving Thessaly to the young son of Constantine, John II Doukas. As he too was underage, Constantine had named the Duke of Athens, Guy II de la Roche (), as his regent and guardian. John II continued his grandfather's pro-Latin policy, maintaining particularly close relations with the Venetians, who imported agricultural produce from Thessaly. In 1308, Guy II de la Roche died. John II Doukas seized the opportunity to sever his dependency on Athens, and turned to Byzantium for support. In early 1309, the Catalan Company, in conflict with the Byzantines, crossed from Macedonia into Thessaly. The arrival of the marauding Company, some 8,000 strong, alarmed John II Doukas. Defeated by the Greeks, the Catalans agreed to pass peacefully through Thessaly to the south, towards the Frankish principalities of southern Greece. After seizing the Spercheios Valley and capturing Salona, the new Duke of Athens, Walter of Brienne, hired the Company for service against Thessaly. Turning back, the Catalans captured Zetounion, Halmyros, Domokos and some thirty other fortresses, and plundered the rich plain of Thessaly, forcing the John Doukas to come to terms with Walter. Walter however tried to backtrack on his deal with the Catalans, leading to open conflict. In the Battle of Halmyros on 15 March 1311, the Catalans crushed the Athenian army, killing Walter and most of the leading nobles of Frankish Greece. After the battle, the Catalan Company proceeded to occupy the defenceless duchy. Thessaly in the 14th and 15th centuries The Catalans continued for a while to hold the parts of southern Thessaly they had occupied and raided the region in the following years, while John Doukas' authority was increasingly enfeebled in Thessaly itself at the expense of the large landholders, who became virtually autonomous, maintaining their own, independent contacts with the Byzantine court. As a result, probably , John too was forced to formalize his relations with the Byzantines, recognizing the Empire's suzerainty and marrying Irene Palaiologina, the illegitimate daughter of Emperor Andronikos II Palaiologos. When John Doukas died in 1318, the southern part of Thessaly was quickly captured by the Catalans of Athens. Between 1318 and 1325, the Catalans took Neopatras, Zetounion, Loidoriki, Siderokastron, and Vitrinitsa, as well as—apparently briefly—Domokos, Gardiki, and Pharsalus. This territory formed the new, Catalan-dominated Duchy of Neopatras. Venice also took advantage of the anarchy in Thessaly to acquire the port of Pteleos. The central and