Our conversation with Allen Guelzo, the Henry R. Luce Professor of the Civil War Era at Gettysburg College and author most recently of Gettysburg: The Last Invasion, published by Alfred A. Knopf. In this interview, Dr. Guelzo discusses the lure of the Battle of Gettysburg as an historian, the “source problem” that goes along with researching such a battle, and the impact of Edwin Coddington’s work on subsequent scholarship. Finally, Guelzo explains why he believes Gettysburg resonates with so many Americans on the eve of the battle’s sesquicentennial.