Towards the end of his life Ivan was partially consoled for his failure in the west by the unexpected acquisition of the kingdom of Siberia in the east, which was first subdued by the Cossack hetman Ermak or Yermak in 1581.
It was proposed, therefore; in 1576, that 6000 families should be registered as a militia under a Polish Hetman for the protection of the country against Tatar raids, and that the remainder of the inhabitants should be assimilated to the ordinary peasants of Poland.
The registered Cossacks objected to being placed under a Hetman not freely chosen by themselves, and those who were not included in the militia objected still more strongly to the prospect of being reduced to the miserable condition of Polish serfs.
But the idea of a retreat was intolerable to him, so he determined to march southwards instead of northwards as suggested by his generals, and join his forces with those of the hetman of the Dnieperian Cossacks, Ivan Mazepa, who had 100,000 horsemen and a fresh and fruitful land at his disposal.
Dissensions among the Cossacks led to the recognition by Turkey of Doroshenko, the hetman of the Sari Kamish, as ruler of the Ukraine; the Zaporog Cossacks, his antagonists, applied for aid to Russia.