Here's a question for you. What if they're right and that we should follow the faith under Sharia Law? If we all did, would it mean the end of terrorism and would the planet live in sustainable harmony?
No. As others have said, they fight amongst themselves all the time. Surely the Middle East is clear evidence of that.
Anyway, I can't see myself praying five times a day and living within their rules, so I certainly wouldn't be living in harmony with them. I suspect that the majority of people wouldn't either.