Great war photographers bring a tremendous sense of mission to their work. Most of them believe the right image seen by enough people at the right time can change the world. Maybe not right away – but in time. Over the past 30 years, the photographer James Nachtwey has covered just about every major armed conflict in the world. He's been shot and wounded more than once, and nominated for the Nobel Peace Prize ten times. We talked with him when he had just put together an exhibition of photos he took in Iraq, Afghanistan, and the place those wars began - Ground Zero on 9/11.