In 1096, an expedition of extraordinary scale and ambition set off from Western Europe on a mass pilgrimage to Jerusalem. Three years later, after a journey which saw acute hardship, the most severe dangers and thousands of casualties, the knights of the First Crusade found themselves storming the fortifications and capturing the Holy City from its Muslim overlords. Against all the odds, the expedition had returned Jerusalem to Christian hands. With its themes of the rise of the papacy, the confrontation between Christianity and Islam, the evolution of the concept of holy war, of knightly piety and religious devotion, the First Crusade is one of the best-known and most written-about events in history. Yet this fascinating and innovative study, Peter Frankopan shifts the paradigm and asks vital questions that have never been posed before. Why was there an overwhelming desire to liberate Jerusalem in the mid-1090s, given that the city had been taken by the Muslims nearly 500 years earlier? What were the causes of the Crusade in the east which provoked such an overwhelming response in the west? What role was played by the Byzantine emperor in Constantinople in the genesis and execution of the expedition? In short, why was there a First Crusade? Rather than concentrating on the pope and the knights of western Europe who have dominated the history of the First Crusade for centuries, Frankopan focuses on Constantinople and the Byzantine Empire. He brilliantly restores the Emperor Alexios I Komnenos to the heart of the story, with a series of catastrophic events in the mid-1090s serving to paint a compelling and strikingly original picture of the expedition to Jerusalem that will change our understanding of the Crusades as a whole.