The Gates of Europe: A History of Ukraine by Serhii Plokhy:
In 1385, in the town of Kreva (now in Belarus), the thirty-three-year-old grand duke of Lithuania, Jogaila, who called himself by God’s grace “Grand Duke of the Lithuanians and Lord of Rus’,” signed a decree that was, in all but name, a prenuptial agreement with representatives of the twelve-year-old queen of Poland, Jadwiga. In exchange for the Polish throne, he agreed to accept Catholicism for himself and his realm and brought about a union of the lands of the Kingdom of Poland and the Grand Duchy of Lithuania. A year later, Jogaila was crowned king of Poland. Another year passed, and in 1387 the combined Polish and Lithuanian forces helped to wrestle Galicia from the Hungarians and attach it once again to the Polish kingdom.
A number of other unions would follow the one negotiated in Kreva, strengthening ties between the two polities and culminating in the Union of Lublin (1569), which created the Polish-Lithuanian Commonwealth. The borders between the kingdom and the duchy were realigned within the commonwealth, transferring most of the Ukrainian territories to the kingdom and leaving the Belarusian ones within the boundaries of the duchy. The union of Poland and Lithuania thus meant the separation of Ukraine and Belarus, and in that regard we can hardly overestimate the importance of the Union of Lublin. It would initiate the formation of the territory of modern Ukraine and its intellectual appropriation by the local elites.