Alternative History

This history is dependent on two changes in European history.  First, Jan Hus did not go to the Council of Constance and thus was not burned at the stake, allowing him to keep his movement from splitting and to spread both into Poland and Hungary and east into Lithuania (Then, much larger than it is now) and the lands of the Ottoman Empire.  Thus the Protestant Reformation happens a century earlier and divisions are along East-West lines rather than North-South.

The second change happens with another person not dying when their supposed to, in this case Henry V.  He lives another 25 years is easily able to squash Joan of Arc and consolidates France and Britain into one realm instead of leaving a one year old child as king of England.  Instead, he has three children and the pious and scholarly Henry is persuaded to abdicate in favor of his more politically and martially inclined brother Richard-Louis I on Henry's death in 1447.  In return Henry is made a bishop and extracts a promise from his brother to lead a crusade against the eastern heresy.   Between consolidating his rule and persuading the Holy Roman Empire to an alliance it took five years and marrying his younger sister Eloise off to Frederick IV of Germany (instead of Eleanor of Portugal in OTL), soon to be Emperor Frederick III  keep that promise.

During this time the Protestants had gained an unlikely ally in the form of the Ottoman Empire who saw them as a way of siphoning believers away from the Orthodox Byzantine Empire and of use as buffer states against the Catholic nations to it's west and so funnels money, equipment and advisors to train their troops.  However Murad stops at actually sending soldiers to support them.  He has the conquest of the Byzantine Empire in mind after all.

When the Crusade finally launches in 1552, the forces of the United Kingdom of Britain and France and the Holy Roman Empire are joined by the Teutonic Knights to Lithuania's north, fueled both by religious fanaticism and greed for Lithuanian territory.