At last Charles arrives on the borders of the Boristhenes, at a small town called Mohilow. This was the important spot where it was to be determined, whether he should direct his march eastward, towards Moscow; or southwards, towards the Ukraine. His own army, his friends, his enemies, all expected that he would direct his course immediately for the capital of Russia. Which ever way he took, Peter was following him from Smolensko with a strong army; no one expected that he would turn towards the Ukraine. He was induced to take this strange resolution by Mazeppa, hetman of the Cossacks, who, being an old man of seventy and without children, ought to have thought only of ending his days in peace: gratitude should have bound him to the czar, to whom he was indebted for his present dignity; but whether he had any real cause of complaint against that prince, or that he was dazzled with the lustre of Charles's exploits, or whether, in time, he thought to make himself independent, he betrayed his benefactor, and privately espoused the interests of the king of Sweden, flattering himself with the hopes of engaging his whole nation in a rebellion with himself.
Charles made not the least doubt of subduing the Russian empire, as soon as his troops should be joined by so warlike a people as the Cossacks. Mazeppa was to furnish him with what provisions, ammunition, and artillery, he should want; besides these powerful succours, he was to be joined by an army of sixteen or seventeen thousand men, out of Livonia, under the command of general Levenhaupt, who was to bring with him a prodigious quantity of warlike stores and provisions. Charles was not at the trouble of reflecting, whether the czar was within reach of attacking the army, and depriving him of these necessary supplies. He never informed himself whether Mazeppa was in a condition to observe his promises; if that Cossack had credit enough to change the disposition of a whole nation, who are generally guided only by their own opinion; or whether his army was provided with sufficient resources in case of an accident; but imagined, if Mazeppa should prove deficient in abilities or fidelity, he could trust in his own valour and good fortune. The Swedish army then advanced beyond the Boristhenes towards the Desna; it was between these two rivers, that he expected to meet with Mazeppa. His march was attended with many difficulties and dangers, on account of the badness of the road, and the many parties of Russians that were hovering about these regions.