The reasons of the unification of Ukraine and Muscovy in 1654 are of great importance for historians. The judicial nature of the union has been the topic of various discussions and polemics. A detailed study of the issue was done by J. Bassarab in 1982.1 The modern Ukrainian historiography has done a lot in studying Khmelnytsky’s reasons for choosing Muscovy’s tsar. In this essay, I focus on another issue: What was the main reason for Tsar Alekseĭ Mikhaĭlovich to take Ukraine under his ‘high hand’? Did he actually think about a ‘reunification’ or ‘gathering of the Rus’lands’ (these ideas being quite popular in Russian historiography)? What was the role of the religious factor, Russian Orthodox Church and, particularly, Patriarch Nikon in the tsar’s decision? Why did Muscovy break the Polyanovka Treaty with Polish-Lithuanian Commonwealth and started a war for Ukraine?