But, while the Czar was thus rapidly increasing his power at sea, the King of Sweden proved himself the strongest on land. He extended his conquests very rapidly in Poland and in the adjoining provinces, and at last, in the summer of 1708, he conceived the design of crossing the Dnieper and threatening Moscow, which was still Peter's capital. He accordingly pushed his forces forward until he approached the bank of the river. He came up to it at a certain point, as if he was intending to cross there. Peter assembled all his troops on the opposite side of the river at that point in order to oppose him. But the demonstration which the king made of an intention to cross at that point was only a pretense. He left a sufficient number of men there to make a show, and secretly marched away the great body of his troops in the night to a point about three miles farther up the river, where he succeeded in crossing with them before the emperor's forces had any suspicion of his real design. The Russians, who were not strong enough to oppose him in the open field, were obliged immediately to retreat, and leave him in full possession of the ground. Peter was now much alarmed. He sent an officer to the camp of the King of Sweden with a flag of truce, to ask on what terms the king would make peace with him. But Charles was too much elated with his success in crossing the river, and placing himself in a position from which he could advance, without encountering any farther obstruction, to the very gates of the capital, to be willing then to propose any terms. So he declined entering into any negotiation, saying only in a haughty tone "that he would treat with his brother Peter at Moscow." On mature reflection, however, he seems to have concluded that it would be more prudent for him not to march at once to Moscow, and so he turned his course for a time toward the southward, in the direction of the Crimea and the Black Sea. There was one secret reason which induced the King of Sweden to move thus to the southward which Peter did not for a time understand. The country of the Cossacks lay in that direction, and the famous Mazeppa, of whom some account has already been given in this volume, was the chieftain of the Cossacks, and he, as it happened, had had a quarrel with the Czar, and in consequence of it had opened a secret negotiation with the King of Sweden, and had agreed that if the king would come into his part of the country he would desert the cause of the Czar, and would come over to his side, with all the Cossacks under his command.