ANSWER: The crusades were produced by the religious and military spirit which prevailed in Europe during the middle ages.
The love of arms and religious fervor were two dominant passions, which, mingled in some way, lent each other a mutual energy. These two great principles, united and acting together, gave birth to the holy war; and carried, among the Crusaders, valor, resignation and heroism of character to the highest degree of eminence.
(History of the Crusades, Michaud, Vol. 1, Intro)
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