The Sultan on hearing of Stephen’s alliance with Hungary sent to demand the tribute, and this being refused he vowed vengeance, but stayed his hand for a short while to attend to other affairs. The despairing King implored help of all his neighbours, and prepared for a last stand. More troops were levied in Bosnia, and envoys were sent to Italy and Croatia to enlist mercenaries.[410] But the support of his people was lacking, and resistance hopeless. Ragusa could not give men, being herself hard pressed, but gave arms and ammunition.[411] Finding himself in desperate straits he sent envoys to Constantinople to offer to pay the tribute once more and ask for a fifteen years’ truce. Mohammed granted this request, fully intending to attack Bosnia at once. The Servian Michael of Ostrovica, who heard the Sultan discussing this treachery, warned the Bosnian ambassadors, but they laughed at him and returned home with the good news. Mohammed then began his northward march with 15,000 horse and countless foot, and let out that he intended to attack Hungary itself, so that Matthew Corvinus should not send help to Bosnia. The army marched through Üsküb to Senice, and an advanced guard under Mohammed Pasha captured Podrinje in Bosnia. The great fortress of Bobovac, which had hitherto resisted all Turkish sieges, was next attacked. It might easily have held out for many months, but the Governor, Knez Radak, a Bogomil who had been converted to Catholicism by force, surrendered it without a struggle. The traitor, however, was beheaded by the Turks, and a large part of the inhabitants made prisoners, including the very envoys who had brought the charter of the truce from Constantinople. The news of the fall of Bobovac caused the most widespread dismay throughout the land, and the Turkish advance was almost unopposed, many of the Bogomil nobles going over to the enemy. In eight days about eighty towns had surrendered. The King fled from Jajce to Kljuć, where he was pursued by Mohammed Pasha and besieged. On a promise that his life would be spared if he surrendered, he gave himself up, and was brought as a prisoner before the Sultan at Jajce, which had also opened its gates to him on the understanding that its inhabitants should be unmolested. The craven King helped to make the conquest all the easier by authorising his governors and officers to surrender (June 1463). The Sultan now wished to complete his conquests by annexing the Herzegovina. Stephen Kosača at first meditated flight to Ragusa, but then determined to hold out for a time, and sent his son, Vladislav, to levy troops on the coast. The Turkish advance through the bare and rocky Karst mountains of the Duchy proved more difficult than was anticipated. Mohammed besieged Blagaj, the Duke’s residence, in vain, captured Kljuć (not the Bosnian town of that name) and Ljubuski, but soon lost them again.[412] A few weeks later he abandoned the scheme and returned to Constantinople.