In 1344, the regency concluded a further alliance with Bulgaria, which required the surrender of Philippopolis  and nine other towns in northern Thrace along the river Evros. Nevertheless, after their occupation, Ivan Alexander refrained from direct action against Kantakouzenos' forces operating in southern and eastern Thrace. At the same time, Momchil, a former brigand whom Kantakouzenos had entrusted with control over the region of Merope in the Rhodope mountains, switched over to the regency. In early 1344, Kantakouzenos was deprived of Umur and the bulk of his army, who had sailed home to repel a Latin attack on his main harbour, Smyrna. On their way, the Turkish force was attacked by the Serbs under Gregory Preljub, but prevailed at the Battle of Stephaniana. Nevertheless, Kantakouzenos was able to ward off joint attacks by Dušan and Apokaukos until Umur returned to his aid the next spring at the head of an army of 20,000 men. Kantakouzenos and Umur raided Bulgaria, and then turned against Momchil. The latter had exploited the power vacuum in the Rhodope, an effective no man's land between the Serbs, Bulgarians and Byzantines, to set himself up as a quasi-independent prince, supported by a substantial force of around 5,000 men. On 7 July 1345, the two armies clashed at Peritheorion. Momchil's army was crushed, and he himself fell in the field. Soon afterwards, Dušan arrived before Serres and laid siege to the city. Rejecting demands by Kantakouzenos to withdraw, a clash appeared inevitable until the murder of Alexios Apokaukos in Constantinople forced Kantakouzenos to direct his attention there.. Who had the largest army?