For a brief moment, towards the end of the 12th century, the Muslim world was politically united under one caliph ruling from Baghdad. This political unity, rare in Islamic history, projected itself on the military plane. In West Asia, the Crusaders were ejected from Palestine, Lebanon and Syria. Salahuddin recaptured Jerusalem in 1187. Four years later, in 1191, Muhammed Ghori of Ghazna...

A divided Islamic world offered feeble resistance to the Crusaders who consolidated their hold on the eastern Mediterranean and imposed their fiefdoms on the region. The Seljuks, preoccupied with defending their eastern flank against the Afghan Ghaznavids, had thinned out their western defenses. The pagan Turkish tribes across the Amu Darya on the northeastern frontiers were a constant menace....

The fall of Jerusalem was the price paid by the Muslims for the continued civil wars brought on by competing Sunni and Shi’a visions of Islamic history. The Crusades, declared in 996, were an intercontinental invasion across a front line extending more than 3,000 miles from Spain to Palestine. At the time, the house of Islam was divided into three households. The Turks championed the...