As "bailli" Guy was charged with the defence of Thessalonica against the Epirotes. Urgent calls for a crusade were made in the West, under the leadership of William VI of Montferrat, but in the event, only a small vanguard under the former regent, Count Oberto II of Biandrate, arrived at Thessalonica in summer 1222. The Latin Emperor Robert of Courtenay () also promised aid and attacked Serres in April 1224, but the siege had to be abandoned following the disastrous defeat of the main Latin army at the hands of the Nicaean emperor John III Doukas Vatatzes () at the Battle of Poimanenon. Left unsupported, the garrison of Thessalonica surrendered in December 1224. Theodore treated the surrendered Latins well, and eventually released most of them, including Guy, from captivity.