The decision of the (Ruthenian) Church of Rus', the "Metropolia of Kiev-Halych and all Rus'", to break relations with the Patriarch of Constantinople and place themselves under the (patriarch) Pope of Rome in 1595-1596 is termed the Union of Brest (Belarusian: Берасьце́йская ву́нія). At the time, this church included most Ukrainians and Belarusians, under the rule of the Polish-Lithuanian Commonwealth. The hierarchs of the Kievan church gathered in synod in the city of Brest composed 33 articles of Union, which were accepted by the Pope of Rome.

The union was opposed by some bishops and prominent nobles of Rus', and perhaps most importantly, by the nascent Cossack (Kozak) movement for Ukrainian self-rule. The result was "Rus' fighting against Rus'," and the splitting of the Church of Rus' into Greek Catholic (also known as Uniate) and Greek Orthodox jurisdictions.