No one knows how many thousands of Iraqis were killed in the war that toppled Saddam Hussein's regime. But if history is any guide, some of the troops who saw combat will come home to face a longer battle -- with their memories. The sights, the sounds and the smells of killing may follow soldiers for years. CNN correspondent Candy Crowley examines the effects of war through the eyes of three combat veterans in "Fit To Kill." An examination of how soldiers cope with killing in combat; and the impact it has on them once they return home. The documentary features insights from military historian David Steighan; retired Lt. Col. David Grossman, author of “On Killing”; neuro-biologist Dr. James McGaugh; and Col. Jim Stokes, an Army psychiatrist.

• Anderson Cooper