The union of Poland with Lithuania, under a dynasty founded by the Lithuanian grand duke Jagiello, dominates is the next major period in Poland's history.

The partnership proves profitable for the Poles, who will play a dominant role in one of the most powerful empires in Europe for the next three centuries.

Poland's unlikely partnership with the adjoining Grand Duchy of Lithuania, Europe's last heathen state, provides an immediate remedy to the political and military dilemma caused by the end of the Piast Dynasty.

At the end of the fourteenth century, Lithuania is a warlike political unit with dominion over enormous stretches of present-day Belarus and Ukraine.

Putting aside their previous hostility, Poland and Lithuania see that they share common enemies, most notably the Teutonic Knights; this situation is the direct incentive for the Union of Krewo in 1385.

The compact hinges on the marriage of the Polish queen Jadwiga to Jagiello, who becomes king of Poland under the name Wladyslaw Jagiello.

In return, the new monarch accepts baptism in the name of his people, agrees to confederate Lithuania with Poland, and takes the name Wladyslaw II.

In 1387 the bishopric of Wilno is established to convert Wladyslaw's subjects to Roman Catholicism.
(Eastern Orthodoxy predominates in some parts of Lithuania.)

From a military standpoint, Poland receives protection from the Mongols and Tatars, while Lithuania receives aid in its long struggle against the Teutonic Knights.