David Perry, a medievalist, writing on CNN:
The idea that contemporary military and terrorist activities in the Middle East embody a new Crusade isn't exactly new. What's startling is that today both supporters of ISIS and radical Christian terrorists have adopted the same language. Both sides are using medieval history to justify their violent intentions. ...But one thing from the history is very clear to me. The modern use of the Crusades to justify violence by either Muslims or Christians is, in fact, modern. The claim that there's an inexorable conflict between Islam and "The West" is not based on history, but rhetoric used by extremists to promote their causes.