 Complete the following phrase:  However, in the times to come, Serbia couldnt rely much on its western allies. Hungary was in the internal crisis of its own, while European courts and church provided no help, organizing instead a series of fruitless councils in Wiener Neustadt, Frankfurt and Mantua. The downfall of Serbia was accelerated after the Fall of Constantinople in 1453 which allowed for the Ottoman sultan Mehmed the Conqueror to turn his efforts to capture the remaining parts of the Balkans and Hungary. The Turks conquered the southern parts of Serbia in 1455, rich in both ores and arable land, including the large cities and mines of Trepča end, especially important and wealthy Novo Brdo, the mother of all cities. Mehmed tried to capture 