In his marvelous new book, 1861: The Civil War Awakening, Adam Goodheart tries to capture what it felt like to live through secession and the opening months of the Civil War, at a time when it wasn’t clear, or destined, that the war would become the bloodiest and most important event in American history. A historian at Washington College and one of the lead authors of the New York Times’ Disunion blog, Goodheart writes especially vividly about photography, so last week I invited him to tour “The Last Full Measure,” a new Library of Congress exhibition of portrait photographs of Civil War soldiers.