The Treaty of Pereyaslav was concluded in 1654 in the Ukrainian city of Pereyaslav during the meeting known as Pereyaslavska Uhoda (Pereyaslav Treaty). The treaty provided for the protection of the Ukrainian Cossack state during the Bohdan Khmelnytsky rebellion, by the Tsar of Muscovy. Participants in the preparation of the treaty at Pereyaslav included the Cossack Hetman Bohdan Khmelnytsky, numerous Cossacks and large visiting contingent from Russia and translators. The original copies of the treaty have perished, and the exact nature of the relationship stipulated by this treaty between Ukraine and Russia is a matter of scholarly controversy.
Whatever the nature of the treaty, the consequences were more clear over time. Major consequences of the treaty included the separation of Ukraine from formerly dominant Catholic Poland, the strengthening of Orthodoxy in the historic center of Ukraine, and the eventual domination of Ukraine by neighboring Orthodox Russia.
In the long run, the consequences for Ukraine were pivotal. Polish colonization and Polonization of the upper class soon became replaced by a systematic process of Russification, culminating in the Ems Ukaz, which banned the Ukrainian language. Also suppressed was the distinct identity of the Kievan Church of Rus': both branches of the Ukrainian Church resulting from the Union of Brest were suppressed.
For Poland, the treaty marked a beginning of sorts of a process of dismemberment leading to complete loss of independence (1795).
For Russia, the treaty eventually led to the acquisition of Ukraine, providing a justification for the ambitious title of the Muscovite rulers, Tsar of the Whole Rus'.
This treaty is seen by Ukrainian nationalists as a sad occasion of the lost chance for Ukrainian independence. Pro-communist and pro-Russian Ukrainian parties celebrate the date of this event calling for the re-unification of Russia, Ukraine and Belarus.
In 2004, after the celebration of the 350th anniversary of the event, the administration of president Leonid Kuchma of Ukraine established January 18 as the official date to commemorate the event, a move which created controversy. Previously, in 1954, the anniversary celebrations included the controversial transfer of Crimea from the Russian Republic to the Ukrainian Republic of the USSR.