On Rhodes the Hospitallers were forced to become a more militarised force, fighting especially with the Barbary pirates. They withstood two invasions in the 15th century, one by the Mamluk Sultan of Egypt Sayf ad-Din Jaqmaq in 1444 and another by the Ottoman Sultan Mehmed the Conqueror in 1480 who, after capturing Constantinople and defeating the Byzantine Empire in 1453, made the Knights a priority target.