In terms of geopolitics, perhaps the most seminal event of the Middle Ages was the successful Ottoman siege of Constantinople in 1453. The city had been an imperial capital as far back as the 4th century, when Constantine the Great shifted the power center of the Roman Empire there, effectively establishing two almost equally powerful halves of antiquity's greatest empire. Constantinople would continue to serve as the capital of the Byzantine Empire even after the Western half of the Roman Empire collapsed in the late 5th century. Naturally, the Ottoman Empire would also use Constantinople as the capital of its empire after their conquest effectively ended the Byzantine Empire, and thanks to its strategic location, it has been a trading center for years and remains one today under the Turkish name of Istanbul. 

Naturally, the Byzantines and Europeans didn't take the Ottoman incursions laying down, and in the generations before the Ottoman victory at Constantinople, European alliances frequently tried to check Ottoman advances, couching their campaigns in the terms of crusading. One of them came near the end of the 14th century, and it presaged what was to come in the 15th century. 

The Ottoman campaigns of the 14th century would provide the context for the events that led to the Battle of Nicopolis. One of the last major crusades was launched in 1396 by Pope Boniface IX, and the timing was perfect for the European kingdoms to unite and form a strong threat to the Ottomans. The 100-year war between France and England was in a state of truce, and King Richard II had just married Princess Isabella of France. Thus, both the English and the Franks would be able to send forces to join in a crusade, and so would Hungary, Bulgaria, Venice, Genoa, Croatia, Wallachia, the Holy Roman Empire, and the Knights Hospitaller. It is estimated that both the Crusader forces and the Ottoman armies consisted of somewhere between 15,000–20 000 men each.