The weekend's shooting reminds us how common assassination attempts on politicians are in our country. And while people spar back and forth about whether Jared Loughner is crazy or whether he is political, there's another conversation to be had: Why does radicalism become attractive to people? And how has the brew of high-voltage rhetoric, accessibility of weapons, and a dash of encouragement lead people to commit atrocious acts that, at times, have drastically changed the course of our nation? Douglas Brinkley is a historian and a professor at Rice University and author of "The Wilderness Warrior" and "The Great Deluge." He brings historical perspective to the tragic event.

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