Radical Islam and Islamic extremism seem to be fairly popular in the past few days (mods, would this maybe make for a good entry in the FAQ?), there have been two similar topics recently that you may want to check out:

• Was radical Islam an issue prior to the 20th century? If not, what was the catalyst that made modern Islam have a large radical following?
• What is the history of (modern) Islamic extremism? When did this all start and why? (x-post from /r/history)

I'm not sure these fully answer your question, but hopefully they'll provide a good starting point. There's always room for more answers.