Abstract: The 1648 Ukrainian Cossack rebellion led by hetman Bohdan Khmelnytsky and ensuing war against the Polish-Lithuanian Commonwealth sparked the beginning of a new era in the history of Eastern Europe. The successes of the rebels were largely due to cooperation between the Orthodox Ukrainian Cossacks and the Muslim Crimean Tatars: from 1648 until 1654 the Crimean Khanate led by Khan Islam Giray III played a crucial role in the success of the Ukrainian Cossacks against forces of the Commonwealth. Throughout most of this tumultuous period the Ottoman Empire and Muscovy avoided being drawn into the Ukrainian-Polish conflict, though by 1654 Moscow was forced to abandon its reluctance to become involved in Ukraine. Mean-while the Ottomans maintained non-involvement in north until the late 1660s. As to the powers that sought to alter the international order in Eastern Europe, Islam Giray and Bohdan Khmelnytsky had different and even conflicting goals and expectations which meant that the Crimean Tatar-Ukrainian Cossack cooperation was doomed to fail. This presentation will analyze the conditions that prompted the Tatar khan and Cossack hetman to cooperate for six years and the factors that contributed to the break-up of their alliance.