The Gates of Europe: A History of Ukraine by Serhii Plokhy:
The turning point in the internationalization of the Khmelnystky Revolt took place on January 8, 1654, in the town of Pereiaslav. On that day, Bohdan Khmelnytsky and a hastily gathered group of Cossack officers swore allegiance to the new sovereign of Ukraine, Tsa Aleksi Romanov of Muscovy. The long and complex history of Russo-Ukrainian relations had begun. In 1954, the Soviet Union lavishly celebrated the tricentennial of the “reunification” of Ukraine and Russia. The implication was that all of Ukraine had chosen at Pereiaslav to rejoin Russia and accepted the sovereignty of the tsar. What actually happened at Pereiaslav in 1654 was neither the reunification of Ukraine with Muscovy (which would be renamed “Russia” by Peter I) nor the reunion of two “fraternal peoples,” as suggested by Soviet historians. No one in Pereiaslav or Moscow was thinking or speaking in ethnic terms in 1654.