 As Demetrios grew more and more pro-Ottoman, especially after Mehmed had promised to marry Demetrios's daughter Helena, the antagonism grew between the two brothers.In January 1459, their rivalry broke out into civil war as Thomas, with the aid of some of the Albanian lords in the Morea, seized a series of fortresses held by Demetrios.The ongoing civil war, and the possibility that Thomas could receive aid from the West since he had proclaimed the war against his brother as a holy war against the muslims, caused Mehmed to invade the Morea in 1460.