Byzantine–Ottoman wars
Consequences

The Fall of Constantinople came as a shock to the papacy, which ordered an immediate counter-attack in the form of a crusade. Only Philip the Good, Duke of Burgundy responded but under the condition that a powerful monarch assist him; however, none would do so. Pope Pius II then ordered another crusade. Again, no substantial efforts were seen by any of Europe's major leaders of the time. This forced the Pope himself to lead a crusade. His death in 1464 led to the disbanding of the crusade at the port of Ancona.

The fall also had many implications in Europe: the influx of Greek science and culture into Europe by those escaping the Ottomans was a crucial factor in catalyzing the European Renaissance.

The failed attempts at defeating the Ottomans at Nicopolis and Varna, the loss of the Holy Land (without Byzantium the Crusades could not re-supply en route) and the lack of a genuine counter-attack led many, including Martin Luther, into believing that the Turks were God's punishment against the sins of Christians:

How shamefully... the pope has this long time baited us with the war against the Turks, taken our money, destroyed so many Christians and made so much mischief!

Nonetheless, by 1529, Europe began to rise to the threat of the Ottomans. Martin Luther, changing his views, wrote that the "Scourge of God" had to be fought with great vigour by secular leaders rather than as Crusades initiated by the papacy.

With the Ottomans' hold on Constantinople de facto recognized by Europe's lack of action, the Ottomans went on to facilitate further conquests in Europe and in the Middle East. Their power finally reached a peak in the mid 17th century. Their success through the Janissaries became their new weakness; due to their conservativism and power, Ottoman reform was difficult to implement whilst European armies became increasingly more resourceful and modernized. As a result, Russian and Austrian attempts to contain the Ottoman threat became more and more a formality until the official dissolution of the Empire after World War I.
detectable_content:number_placeholders[5], detectable_format:constrained_response