The Thirteen Years War — HistoryCentral

In the mid-seventeenth century the Cossacks of Ukraine, led by Hetman Bogdan Khmelnytsky, rose in a great rebellion against the rule of the Polish-Lithuanian Commonwealth. Seeking a powerful protector against Poland, Khmelnytsky turned to Russia, and by the Pereiaslav Agreement of 1654 he accepted the sovereignty of the Russian tsar, setting aside hopes of full Ukrainian independence in favor of Russian overlordship.

Russia's acceptance of the Cossacks' allegiance brought it into direct conflict with Poland, which was determined to maintain its control over Ukraine. The result was the Russo-Polish War, often called the Thirteen Years' War, which began in 1654 and was fought across Ukraine, Belarus, and Lithuania. The prolonged conflict drained both states, and Poland was further weakened by a simultaneous Swedish invasion known as the Deluge.

The war finally ended with the Truce of Andrusovo in 1667, which partitioned Ukraine between the two powers along the Dnieper River. Russia gained the eastern bank together with the city of Kiev, while Poland retained the western lands. The settlement marked a decisive shift of power eastward toward Russia and began the long process by which much of Ukraine was drawn into the Russian orbit.