"The Crusades were the first in a series of major efforts by Europe to slow the expansion of the Muslim world at the Christian Middle East's expense.
The final results were a unifying of Islam over the remains of the Christian states in northern Africa and most of the Middle East. Much of the Byzantine Empire had fallen to them also.
The argument is that the Crusades slowed the Muslim expansion (while unifying it), brought some ideas back to Europe about national identity, and most importantly weakened Byzantine enough that the Muslims were able to go on and destroy it. This would set the stage for even more European-Islam wars until the height of Islam at the siege of Vienna in the 17th century.
source = https://answers.yahoo.com/question/index?qid=20080310161345AAN3Uor
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