Whilst Charles, Henry, and Francis were thus employed wasting each other's strength, the Turkish sultan, Solyman the Magnificent, invaded Hungary with a large army, and took Belgrade. Encouraged by this success, he besieged the Island of Rhodes, then the seat of the Knights of Saint John of Jerusalem. The Grand Master of Villers de L'Isle Adam sent imploring messages to the powers of Europe begging for assistance. Adrian VI. did his utmost to persuade Charles and Francis to forget their quarrels and join forces in saving Rhodes, then the chief bulwark of Christianity in the East. On March the 3rd, 1523, he wrote to Charles exhorting him and all Christian princes to make peace with one another, and wage a common war against the Turks. He complains that 'so far all his exhortations have been fruitless, and the Turks have conquered Belgrade on one side, and it is said they have taken Rhodes on the other. There is no doubt that the Turks will continue their conquests in Hungary (where the emperor's sister Mary is queen), as well as in the Mediterranean, till they have rendered themselves masters of the whole of Europe. This danger can only be averted by a reconciliation of all Christian princes....' The Pope ends by saying that he has written in the same sense to the Kings of France and England.[83] [83] _Calendar of Spanish State Papers_, vol. ii. But the rival princes turned a deaf ear to all these entreaties, and after six months of incredible courage, patience, and bravery on the part of the garrison, the gallant little band of knights were forced to capitulate, and the town was razed to the ground. When too late, Charles, Henry, and Francis, ashamed of their conduct, tried to lay the blame of this misfortune on each other, and Charles, by way of reparation, gave the Knights of Saint John the Island of Malta, which from henceforth became the chief home of their order. The year 1523 was marked by the revolt and conspiracy of the Constable of Bourbon, a powerful and accomplished French nobleman descended from the Montpensier branch of the Bourbon family, who through his marriage with Suzanne, daughter and heiress of the Duke of Bourbon, had acquired the wealth and honours of that powerful house. Francis I. on his accession had made him Constable of France, and treated him with every mark of favour.