The Teutonic Knights, then led by Ulrich von Jungingen, attacked Lithuania and Poland because the ruler of both places, Jogaila, was a recent convert from paganism to Roman Catholicism and the Knights believed (or claimed to believe) that his conversion was not genuine and the conversions of the Lithuanians in general were fraudulent.

Asking what the Pope thought about this is actually a difficult question because, during this time, there was more than one pope. When the Battle of Grunwald occurred in 1410, there were three men all claiming to the pope and most other European kings were focused on that rather than what the Teutonic Knights were doing. Two notable exceptions were the Holy Roman Emperor, Sigismund (also the king of Hungary) and his half- brother, Wenceslaus, who were technically allies of the Teutonic Knights, but at that point, were distracted by other matters.