Early in his reign he adopted a much bolder attitude toward the Christian Powers of Europe than Murad had thought prudent. To a deputation from Italy asking for a renewal of commercial privileges, he replied that when he had conquered Hungary he intended to ride to Rome, and there give feed to his horse with oats on the altar of St. Peter’s. His treatment of his Christian subjects was much harsher than that of his predecessors. Bayezid followed up his father’s great victory at Kossova over the Serbians, and compelled Stephen, the successor of Lazar, to sue for peace. The terms of the treaty then agreed to were very moderate. Instead of being incorporated in the Ottoman Empire as Bulgaria had been, Serbia was to be an autonomous State, under vassalage to the Ottoman Empire, paying tribute in money, and bound to provide and maintain a contingent of five thousand soldiers at the disposal of the Sultan. Stephen, its prince, also gave his sister, Despina, to the Sultan as an additional wife. He most loyally carried out his promises to Bayezid. In the great battles of Nicopolis against the Hungarians and the crusaders from Western Europe, and of Angora against Timur, the Serbian contingent fought with the utmost bravery, and there were no more loyal soldiers in the Ottoman ranks. Having come to terms with Serbia, Bayezid marched southwards with his army, and took up a menacing position near to Constantinople, where the aged and feeble John Palæologus still reigned, supported by his son Manuel as co-Emperor. By threatening to promote the cause of Andronicus, whose eyesight had not been quite extinguished, after his mad rebellion against the Emperor, the Sultan compelled the two Emperors to sign a treaty, under which the remnant of the Greek Empire became an abject vassal State to that of the Ottomans. The Emperors promised to pay an annual tribute of thirty thousand pieces of gold and to supply a contingent of twelve thousand men to the Ottoman army to be at the disposal of the Sultan for any purpose he might design. They also undertook to surrender to the Ottomans the stronghold of Philadelphia, the only remaining possession of the Byzantine Empire in Asia Minor. When the officer in command of that city refused to surrender it, Bayezid insisted on the Greek Emperor employing his contingent in capturing his own city, and on his leading the assault on it, with the aid of his son Manuel, for the purpose of handing it over to himself, their nominal ally, but crafty and designing foe. It would be difficult to imagine a lower depth of humiliation and cowardice than that to which the Emperor and his son thus descended. These public humiliations were aggravated by a domestic one. Bayezid, having captured at sea a vessel bringing a foreign princess as a bride for Manuel, took a great fancy for the lady, and insisted on her entering his own harem.