The Battle of Nicopolis took place on 25 September 1396 and resulted in the rout of an allied crusader army of Hungarian, Croatian, Bulgarian, Wallachian, French, Burgundian, German, and assorted troops (assisted by the Venetian navy) at the hands of an Ottoman force, raising the siege of the Danubian fortress of Nicopolis and leading to the end of the Second Bulgarian Empire. It is often referred to as the Crusade of Nicopolis as it was one of the last large-scale Crusades of the Middle Ages, together with the Crusade of Varna in 1443–1444. By their victory at Nicopolis, the Turks discouraged the formation of future European coalitions against them. They maintained their pressure on Constantinople, tightened their control over the Balkans, and became a greater threat to central Europe.
There were many minor crusades in the 14th century, undertaken by individual kings or knights. Most recently there had been a failed crusade against Tunisia in 1390, and there was ongoing warfare in northern Europe along the Baltic coast. After the Battle of Kosovo in 1389, the Ottomans had conquered most of the Balkans and had reduced the Byzantine Empire to the area immediately surrounding Constantinople, which they blockaded from 1394 on.
In 1393 the Bulgarian tsar Ivan Shishman had lost Nicopolis — his temporary capital — to the Ottomans, while his brother, Ivan Stratsimir, still held Vidin but had been reduced to an Ottoman vassal. In the eyes of the Bulgarian boyars, despots, and other independent Balkan rulers, the crusade was a great chance to reverse the course of the Ottoman conquest and take back the Balkans from Islamic rule. In addition, the front line between Islam and Christianity had been moving slowly towards the Kingdom of Hungary. The Kingdom of Hungary was now the frontier between the two religions in Eastern Europe, and the Hungarians were in danger of being attacked themselves. The Republic of Venice feared that Ottoman control of the Balkan peninsula, which included Venetian territories like parts of Morea and Dalmatia, would reduce their influence over the Adriatic Sea, Ionian Sea, and Aegean Sea.