In A World on Fire, bestselling biographer Amanda Foreman traces turbulent Anglo-American relations during the Civil War.

This massive history is very different from your first book, Georgiana: Duchess of Devonshire. What drew you to the subject?

Actually, Georgiana was the interloping subject. I'd gone to Oxford to do graduate studies in the history of the slave trade, but I came across Georgiana's letters, gave up that thesis, and wrote one on her instead. When I learned that Georgiana's great-nephews supported opposite sides in the American Civil War, I knew this would be the perfect sequel. It was a fascinating microcosm of British attitudes, and it brought me back to something I'd always wanted to finish.

Your father is American, your mother English; you were raised and educated in both countries. Was that dual perspective valuable?

Absolutely! When I started reading British books about Anglo-American relations during the Civil War, I saw this complete disconnect between what historians read on the page—someone's letters, for example—and the realities of Washington at that time. American histories were the same; they had these mad ideas about how Parliament worked, or what people really meant when they said A, B, or C. All my life I felt simultaneously deracinated and rooted in both places, and now it's my greatest strength: I'm culturally bilingual.

How did you go about interweaving the individual stories of British volunteers on both sides with the broader history of diplomatic relations and the progress of the war?

That was the most rewarding aspect for me. It was difficult to have multiple viewpoints and keep the narrative moving, but then I thought of how music is written. I tried to construct a narrative symphony with my story lines, so that readers don't really notice as themes and melodies shift from strings to drums to flute, because the development at all times takes you forward.

Do you think you're taking a commercial risk with such a long, complicated text?

A massive risk, especially since I took 12 years after Georgiana was such a success! In fact, my first publishers in England fired me because they didn't want to take that risk. But I was lucky, because I went to Stuart Proffitt at Allen Lane, who's an outstanding editor. He loved this project, and I worked really hard on it with him and with Susanna Porter at Random House. It worked out extremely well, because they are so different. Susanna's specialty is historical fiction and female biography, which is why I had her for Georgiana. She would say, "You know what? This is really boring. I'm lost here, and I have no idea what you're talking about." While Stuart would say, "You know, I don't think you followed through very well on your argument about the battle of Gettysburg." They read it with two different sets of eyes, and you never get that; I am so grateful.