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, where I interviewed him about the images, and about his book. “The Last Full Measure” displays more than 300 photographs from the Liljenquist Family Collection: Most are portraits of individual soldiers in uniform, and most of those soldiers are unidentified.

Slate: Americans have a sense of the whole scope of the Civil War, but what is it about the year 1861 that we miss when we look back and see the Civil War as a single complete event?

Goodheart: We tend to think of the Civil War in a very fatalistic way, as a sort of vast national martyrdom. It’s hard to recover the fact that things could have turned out very, very differently. It’s also hard to grasp the idea that Americans at the beginning of the war had no sense of the way that things were going to unfold. When I wrote the book, I really wanted to recover that moment of uncertainty and change – to convey how people, then and now, experience history not as seen from 30,000 feet as the History Channel might, but as a barrage of individual events coming at them, alternately thrilling and terrifying.

Slate: What’s an example of a moment in 1861 where the Civil War wouldn’t have had to unfold the way it did, or where secession wouldn’t have had to unfold the way it did?

Goodheart: The greatest blunder that the Confederacy made in the entire Civil War may have been opening fire on Fort Sumter this week 150 years ago. In winning that fort, they may have lost the entire war. What I mean is: Here was this little besieged garrison, 60 soldiers and a brass band, and basically a worthless 2-acre piece of federal real estate on an island in the middle of Charleston Harbor. They were being starved out. The Lincoln administration simply wanted to send provisions to resupply this fort, and the Confederacy opened fire when they did because they didn’t want provisions coming in. Lincoln famously said, “Fort Sumter fell and in so doing did more service than it could have otherwise,” by which he meant that it made the Confederacy inhumane aggressors in the eyes of the nation and in the eyes of the world.

The Confederates had no good strategic reasons for taking that fort, and if they had simply, as a humanitarian gesture, let these Union troops be resupplied with food, I think they would have had time to make the Confederacy seem more and more credible as a nation. New nations have a way of becoming more credible as time passes—they get things like judicial systems, postal systems, diplomats—and if that had happened, if they had simply refused to fire the first shot, I don’t think that Lincoln would have allowed the first shot to be fired, and quite possibly the Confederacy would have gained legitimacy as a nation.

Slate: What about once the war started? Is there a way the war could have unfolded differently once it had begun?

Goodheart: If firing on Fort Sumter was Jefferson Davis’ biggest blunder, Lincoln made his biggest blunder just three days later. After Fort Sumter had fallen, there was an incredible patriotic outpouring across the North—it was like nothing in American history except possibly the attack on Pearl Harbor and the attack on 9/11. There were literally probably over a million American men of military age across the North who were ready to sign up and fight. Men were mobbing recruiting stations. Instead Lincoln called for only 75,000 troops, which disappointed a lot of people. Lincoln didn’t call up all those troops, and the Union mobilized much more slowly than the Confederacy did and missed a chance to strike a decisive blow at the South.

Slate: One of the most entertaining chapters in 1861 is about James Garfield, the Ohio university professor and state legislator who would go on to become a Union general and eventually president of the United States. Who is Garfield and why is he interesting at this moment when the war starts?

Goodheart: I wanted to get inside the brain of an ordinary young American in the North. Of course Garfield turned out to be President of the United States, but at this time he was just a lowly state legislator in Ohio who had come up from poverty. He was the last president to be born in a log cabin. Because he later became president, every scrap of paper, practically, was saved, and he was a prolific writer, diary keeper, he saved every page of his lecture notes—so you can really get inside him in a way that you can’t with other Americans from the time who didn’t become president. And Garfield exemplified the romantic longings of the time, as well as a political ambivalence that then turned into very passionate feelings for the Union and against slavery.

Slate: What’s the 1861 sense of who Abraham Lincoln is? How much do Americans know about him at the start of the war?

Goodheart: Lincoln was the least known figure ever to assume the presidency until then, and I think we forget just how inexperienced he was. He was a one-term Congressman from Illinois, and hadn’t even set foot in Washington in a dozen years when he came to the White House. He was a cipher. Everyone was speculating on what he would do, and nobody thought that Lincoln would be the preeminent power in the White House. Most people thought it was going to be William Seward. We tend to look at Lincoln now from the perspective of this fully realized figure who was martyred four years later, but Lincoln in 1861 was very much groping his way and in many instances blundering his way through this crisis. He made a lot of mistakes. But it’s also this moment right at the beginning of the war when you see him stepping into his own for the first time and making a couple of political masterstrokes that bespoke the kind of leader he would become.

Slate: I read Ulysses Grant’s memoirs recently, and one of the most astonishing things I learned was that in 1861, Grant was a store clerk. Four years later he is the most important man in the country.

Goodheart: Grant is a shop assistant. William Tecumseh Sherman is the president of a streetcar company. They’re both in St. Louis, incidentally, at the same time, as the war begins, and Grant is padding around looking for a commission and not getting one. So the war was also a revolution in the sense of bringing an entirely new group of Americans to power and to prominence while the great figures of the previous era, the Millard Fillmores, the Franklin Pierces, the James Buchanans, sank into obscurity.

Slate: I recently heard Yale Professor David Blight say that there have been more books published on the Civil War than there have been days since the Civil War ended—more than 70,000 of them. Why does the world need another one?

Goodheart: Another figure is that there have been more Civil War books published than there were soldiers at the battle of Bull Run. But I feel like the Civil War is one of those great stories that can just keep being visited and revisited. It’s famously been compared to the Iliad and the Odyssey, in that there’s a new way to narrate it in each generation.

I came to the Civil War very much informed by my experience with 9/11: being here in Washington and seeing mobs of people fleeing from the area of the White House, and people standing on the street corners looking at the column of smoke rising from the burning Pentagon. That moment when just everything changed, when things that had been certainties the day before suddenly evaporated into thin air, was something I think we have a new appreciation of in our times.

Slate: Did everything change suddenly for Americans in 1861, or had they felt secession and the war coming with a creeping sense of inevitability for the previous 20 years?

Goodheart: When Lincoln’s Secretary of State William Seward called it “the irrepressible conflict,” he was widely attacked in both the North and the South, because people still believed in compromise. They still believed in the American political system to work things out. They still believed in white solidarity trumping the politics of slavery. So the bombardment of Fort Sumter really was a revolutionary moment, when many people’s politics changed overnight, many people were ready to go to war in a way that they hadn’t been just 24 hours earlier. Whitman expressed it very well—he said, “News raced through the land as if by electric nerves,” and then he also said, “The Negro was not the chief thing. The chief thing was to stick together.”

Click to view a slide-show essay featuring portraits of Civil War soldiers.