Huniades, who had been living meanwhile retired in one of his castles, now complied with the king's wish without difficulty or hesitation, and again assumed the highest military command. Instead of seeking how to revenge himself after the manner of ordinary men, he only thought of the great enemy of his country, the Turk. And indeed, as it was, threatening clouds hung over the horizon in the southeast. A new sultan had come to the throne, Mohammed II., one of the greatest sovereigns of the house of Othman. He began his reign with the occupation of Constantinople (1453), and thus destroyed the last refuge of the Byzantine Empire. At the news of this event all Europe burst into a chorus of lamentation. The whole importance of the Eastern Question at once presented itself before the nations of Christendom. It was at once understood that the new conqueror would not remain idle within the crumbling walls of Constantinople. And indeed in no long time was published the proud _mot d'ordre_ "As there is but one God in heaven, so there shall be but one master upon earth." Huniades looked toward Constantinople with heavy heart. He foresaw the outburst of the storm which would in the first place fall upon his own country, threatening it with utter ruin. Huniades, so it seemed, was again left alone in the defence of Christendom. The approaching danger was delayed for a few years, but in 1456 Mohammed, having finally established himself in Constantinople, set out with the intention of striking a fatal blow against Hungary. On the borders of that country, on the bank of the Danube, on what was, properly speaking Servian territory, stood the fortress of Belgrade. When the danger from the Turks became imminent, the kings of Hungary purchased the place from the despots of Servia, giving them in exchange several extensive estates in Hungary, and had at great expense turned it into a vast fortress, at that time supposed to be impregnable. Mohammed determined to take the place, and to this end made the most extensive preparations. He led to the walls of Belgrade an army of not less than 150,000 men. The approach of this immense host so terrified the young king that he left Hungary and took refuge in Vienna along with his uncle and counsellor, Czillei.