At the time of his great achievement, Mohammed was only twenty-five years old. He publicly announced that he had reached maturity by decapitating the Grand Vizier Khalil, the tutor set over him by his father, who was suspected of treasonable communications with the Greeks during the siege. He made it plain, also, that there was to be no repose from war after the taking of the capital, the Servians being the first to experience his heavy hand. Brankovitch’s fidelity as a vassal proved no protection to him; for Mohammed wrote claiming his kingdom. In terror the Servian prince fled to Hungary to secure the aid of Hunyadi. The war that followed was hotly contested, with the result that in 1454 the Sultan agreed, on the basis of the large tribute of 30,000 ducats, to recognize Brankovitch. But this peace was not observed, for the conqueror appeared the next year and took Novoberda. Hunyadi, against whom bitter foes were working at the court of the King of Hungary, had only the support of the Wallachian princely house. When Belgrade was attacked by Mohammed, in May, 1456, only 3000 Christian soldiers were ready to oppose him. When the siege really began, however, 200 boats appeared before the city, containing many thousand men of various nationalities, whom the Franciscan monk, John of Capistrano, had drawn to the crusading cause by his protracted and widely extended journeys in Western Europe. Though over seventy years old, he had displayed remarkable energy, and he was honored by the defenders of Belgrade as a holy apostle. On July 15 the two welcome allies took possession of the castle, as the city had not yet been cut off from the outside. The first stage of the defense was the defeat of the Turkish flotilla on the Danube; some vessels were sunk and others were captured, so that entrance into the town by water was made safe. In the attempt to storm the defenses made by the Janitschars, who advanced in small divisions, hardly 600 survived; three times Hunyadi, sallying from the castles, forced back the assailants. Capistrano’s crusaders proved too much for the Sultan’s trained troops; marching right up to the guns and careless of the havoc caused by the cannon fire, those who took part in the sortie cut down the Turks and threw the cannon into the water and ditches. If the crusaders had not stopped on the way to plunder, they would have broken through the Sultan’s own bodyguard. As it was the Ottomans were able to withdraw safely from their camp; but they lost some of their best captains, among them Aga, who was killed while protecting the Sultan, who escaped with an arrow wound.