This document, known as the Union of Krewo (also, the Act of Kreva), was issued on August 14, 1385 by Władysław II Jagiełło (Jogaila), Grand Duke of Lithuania, as the outcome of the negotiations he had conducted with the Polish lords acting on behalf of the child queen Jadwiga (Hedwig), and Jadwiga’s mother Elżbieta Bośniaczka (Elisabeth of Bosnia), Queen of Hungary. The deed established the relationship between the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania, and initiated the long-lasting union between the two countries.
The need for a Polish-Lithuanian union arose in the late fourteenth century around the threat posed to both the Crown and the Grand Duchy by the expansion of the Teutonic Order. The baptism of Lithuania was meant to deny the Teutonic order any pretext for invading the country, to provide the basis for an alliance between Poland and Lithuania, and buttress the rule of the Lithuanian magnates over the Ruthenian population, which was numerically predominant in the Duchy.
Together with all the people of his lands, Jagiełło chose to accept baptism in the Roman Catholic rite, release all Christian captives from Lithuanian bondage, strive to regain the lands lost by the two countries, and also to adjoin [applicare in the Latin text] his Lithuanian and Ruthenian territories to the Crown of the Kingdom of Poland. The latter promise became the seedbed of a dispute between Jagiełło and his first-cousin Witold (Vytautas), who ruled in Lithuania. Moreover, in the following decades this dispute embroiled the Polish and Lithuanian lords, as well. To this day the interpretation of the word applicare remains a subject of controversy between Polish and Lithuanian historians. Polish scholars read the term as describing the intention to incorporate the lands of the Grand Duchy of Lithuania into the Polish Crown, while Lithuanian scholars argue that it expressed the clear intention of maintaining a loose federation of independent states, such as typified the Polish-Lithuanian union until the year 1569.