At the beginning of the 13th century Lithuania was a backward country. Lithuanian tribes were pagan (they worshiped grass-snakes and oaks) and illiterate. They were often at war with each other. At this time the Roman Pope sent German crusaders to Lithuania with the aim of capturing new territories and spreading Catholicism in the area. That threat caused Lithuanians to stop quarrels and unite. In the mid-13th century the Lithuanian tribes were united under the leadership of Prince Mindovg. He defeated a German army and burnt all the prisoners to thank the Lithuanian gods for the victory. Having a powerful army the Lithuanians decided to conquer nearby lands. In the 14th century they moved first into Belarus and then to Ukraine.

At this time Ukraine was under the control of the Golden Horde, the state formed by the Mongol-Tatars in the 13th century.[1] By the mid-14th century the Golden Horde was not as powerful as in the 13th century since it was in a process of disintegration. In 1362 at the Battle of Blue Waters (Suni Vody) the Tatars were defeated by the Lithuanians and Ukraine became part of the Grand Principality of Lithuania. The Lithuanian state was the biggest in Europe and included, besides Lithuanian lands, also the territories of Belarus, Ukraine,[2] and significant parts of Russia (as far as the Oka River with such cities as Viazma and Smolensk).

The Lithuanians were culturally inferior to the Ukrainians as they were pagan and illiterate. So they quickly adapted to a more developed Ukrainian culture. Many of the upper class Lithuanians adopted Orthodox faith and intermarried with Ukrainian nobles. The Ruthenian language (common for Ukrainians and Belarusians at the time) became the official language of government. Ukrainian and Belarusian laws were used to form the basis of the Lithuanian law. Having powerful enemies (Moscow Principality on the east and the Teutonic order of German knights on the west) the Lithuanians were interested in good relations with the Ruthenian (Ukrainian/Belarusian) majority.[3] That is why a popular slogan of the Lithuanian rulers was “We do not change the old traditions.” The Lithuanian army was well organized and strong. It adopted modern military equipment and tactics from German crusaders. Lithuanians built a number of powerful fortifications in Ukraine after the model of German fortresses and castles.