Sigismund understood, and accepted the challenge. In 1392, he invaded Bulgaria, won an initial battle from the Osmanlis, who would have been annihilated had it not been for their new allies, the Wallachians, and, after a long siege, took Nicopolis on the Danube.[462] By this time Bayezid was able to send a large army into Bulgaria. When Sigismund realized how numerous were the forces coming against him, he saw that his victory bade fair to be nothing more than the acquisition of a prison. Before the Osmanlis could surround him, he wisely abandoned Nicopolis. The retreat became a rout.[463] It was on the return from this expedition that Sigismund met Elisabeth Morsinay, in the county of Hunyadi. From their union was born the great champion, who, while his imperial father was engrossed in theological disputes and the complex interests of the empire, battled bravely against Mohammed I and Murad II. The expedition of 1392 demonstrated to Sigismund that Bayezid was a foe worthy of a European ruler, that he must be checked if Hungary were to be saved, and that the Hungarians could not again take the offensive against the Osmanlis without aid from western Europe. For the pretensions of Louis to the overlordship of the Balkan States, and the heartless propaganda of the Catholic faith, thinly disguising Louis’s inordinate ambitions, had turned the Balkan peoples against Hungary and ‘crusaders’ from the west. They chose rather to stand on the side of their Moslem enslavers. Sigismund’s invasion of Bulgaria determined Bayezid to put an end to the arrangement concluded just before Kossova between Murad and Sisman. Bulgaria, like Thrace and Macedonia, was to be an integral part of the empire, and to become converted to Islam and ottomanized, in so far as that was possible. For Sisman, who had re-established himself in his old capital, was too uncertain an ally to be trusted in the event of another Hungarian invasion. In the spring of 1393, an army under Soleiman Tchelebi, Bayezid’s oldest son, to whom this was the first command, surrounded Tirnovo. The bulk of Soleiman’s army was composed of Macedonian Christians and renegades of the first generation. In midsummer,[464] after a three months’ siege, Tirnovo was taken by storm from the side of the old castle, which is still, in part, standing.[465] The inhabitants who escaped fire and sword were carried into captivity in Anatolia. Among them was the patriarch Euthymius.[466]