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.

In 965 or 966 he stayed in Magdeburg at the court of the newly crowned Roman Emperor, Otto I of Germany, with whom he talked, which may prove his visit had a parliamentary character. It is also probable that he learned in those court circles of the existence of an authority not known to him before, the “land of Mesko”. The content of the note does not indicate that he was to visit them personally. He wrote down the acquired information in his account, which has not been preserved in whole to this day, and which is known to us due to its use by Arab compilers: chronicler Al-Bakri in the 11th century (in his work entitled Book of Highways and of Kingdoms) and cosmographer Qazwini of Damascus in the 13th century (Book of Monuments of the Countries).

Since the 1878 edition prepared by W. Rosen and A. Kunik, the text of the Ibrahim ibn Yaqub account has been eagerly used as a source describing the functioning and organisation of the state of Mieszko I, including valuable information about the military in the early-Piast monarchy. According to Ibrahim, the strength of this largest of the Slavic countries state relied on effective and well-managed military units, which comprised, as estimated, 3000 men.