Historian James McPherson is a Professor of American History at Princeton University. He's written eleven books about the Civil War, including his Pulitzer Prize winning book, "Battle Cry of Freedom." His latest book is "For Cause & Comrades: Why Men Fought in the Civil War" (Oxford University Press). Drawing on 25,000 letters and 250 private diaries, McPherson looks at why so many soldiers willingly risked their lives to fight in the war.