How did the Ottoman Turks change the goals of later Crusades?

The rapid expansion of the Ottoman Turks into Christian Europe fundamentally altered the nature and objectives of the later Crusades. As the Ottomans conquered the Balkans, much of Greece, and effectively surrounded Constantinople by the end of the 14th century, the focus of these military expeditions shifted significantly.1

Rather than being aimed at the conquest of faraway Palestine or the Holy Land, the later Crusades became desperate, defensive attempts to protect Europe itself from the advancing Muslim empire. Major efforts, such as the Crusade of Nicopolis and the Crusade of Varna, were organized specifically to repulse the Turkish advance and provide aid to threatened regions like Hungary and Constantinople. This transition marked a move from offensive campaigns of territorial reclamation to a struggle for the survival of Christian territories within Europe.123