God's Battalions: The Case for the Crusades, by Rodney Stark.
Mr. Stark is a brave man to lay out an argument explaining the rationale behind the Crusades, and he's very observant. Indeed, nearly everything I've ever read on the Crusades left me with the impression that the European Crusaders were blood-thirsty, expansionist, war-mongers intent on colonizing the Middle East because, well, that's just what European do.
Steven Runciman's 3-volume A History of the Crusades, for example, reads like a polemical condemnation of all things European set against the lights of a superior Islamic culture. Rodney Stark corrects that, and it is long overdue. He begins by pointing out that much of North Africa, the Middle East and southern Europe were Christian and Jewish for hundreds and hundreds of years before Muhammad was informed by Allah, via the archangel Gabriel, that it was his holy duty to tax, enslave and/or murder non-Muslims. God's Battalions takes to task historians like Runciman, James Carroll, and Karen Armstrong by providing a context to the Crusades.
A book review is not the place to retrace the course of the Crusades, but a partial listing of Muslim conquests from Stark's narrative should give interested readers some idea as to why Europeans at last reacted to the unrelenting Islamic onslaught. Muslims attacked:
Syria - 633
Damascus falls - 635
Byzantine forces abandon Syria following the Battle of Yarmuk - 636
Persia is invaded by peace-loving Muslims - 636
Ctesiphon a city in modern-day Iraq is destroyed - 636
Armenia invaded - 636
Pakistan invaded - 636
Palestine invaded - 636
Jerusalem surrendered - 638
Egypt attacked by peace-loving Muslims - 639-640
Nikiou, Egypt taken, its inhabitants slaughtered - 640
Alexandria, Egypt briefly taken - 641
Alexandria, Egypt retaken, its inhabitants slaughtered - 645
Cypress invaded - 649 falls 653
Sicily invaded - 649
attacked again - 652
attacked again - 667
attacked again - 720
attacked again - 827
Palermo falls 831, Syracuse falls 878, Taormina falls 902
Rhodes invaded - 649 falls 672
Carthage razed and all its inhabitants slaughtered by peace-loving Muslims - 705
Spain invaded - 711
Sardinia falls - 809
Majorca falls - 818
Crete falls - 824
Malta falls to peace-loving Muslims - 835. And on and on and on. Students of the Crusades
should add Mr. Stark's work to their libraries.