In 1682 the population in that part of Hungary which was under the rule of the Emperor Leopold revolted against his bigoted tyranny. Kara Mustapha thought that this afforded an opportunity for attacking Austria. He seems also to have been inflated with ambition to create a kingdom for himself. He collected an enormous army at Adrianople, and in the spring of the following year, 1683, he crossed the Danube at the head of two hundred and seventy-five thousand men, without counting a horde of irregular Tartars and camp followers. He met with little resistance in his march northwards till he reached the walls of Vienna at the head of two hundred thousand men. The Emperor, on his part, was very ill-provided with troops to meet this enormous host of invaders. He had no more than thirty-five thousand men under arms. Of these, eleven thousand were left to garrison Vienna, and the main body was quite insufficient to meet the Turks in the field. In his peril the Emperor appealed for aid to Sobieski, the King of Poland. The Poles had very recently concluded peace with the Turks. But this made no difficulty. Sobieski undertook by treaty to send an army of fifty thousand men in support of the Emperor. There was a clause in the treaty of a significant character. It was not to be annulled by any future dispensation of the Pope. The Polish army, however, was at some distance and could not reach Vienna in less than eight weeks. There can be little doubt that if Kara Mustapha had pressed the siege with vigour Vienna must have fallen before the arrival of the Polish army.