Sometimes, people will bring up the old hoary argument about the crusades, to point out that Islam isn't the only religion that advocates violence. This perception is clouded in ignorance.
In an excellent 5 part video, Robert Spencer will point out the facts about why Islam should be on everybody's mind and a major issue of concern and alarm.
Video #1 will link you to the other 4 videos that make up this lecture. It is well worth watching the whole series.