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When Jay Winik began to research April 1865: The Month That Saved America, he was no confirmed Civil War obsessive; he had been inspired by his career as a congressional aide specializing in international relations.

During the latter half of the ’80s, Winik, now 44, worked for two leading Democrats—the late Rep. Les Aspin of Wisconsin and former Sen. Charles Robb of Virginia—on military and foreign-affairs matters. He spent time in war-torn Nicaragua, El Salvador, and Cambodia, and visited Poland, Hungary, and Czechoslovakia during the waning days of communism.

After leaving government, Winik settled down in his Chevy Chase, Md., home to write a history of the Cold War. That book, On the Brink, was published in 1996—and unexpectedly sowed the seeds for April 1865. “In my research, I started reading about the Civil War to get some historical perspective,” he says. “It turned out to be far more interesting than I expected.”

Winik focused on a key question inspired by his experiences overseas: Why did America’s civil war, unlike so many other conflicts, end relatively quickly and in a way that laid the foundation for future cooperation? To answer that question, Winik zeroed in on the fateful final month of the conflict. In April 1865, “Lincoln was terrified of a guerrilla war,” Winik says. “He feared that Lee could take to the hills for the next 20 years. The Palestinian intifada shows what that strategy could have been like.” But within a month, the war was over.

Winik credits towering leadership on both sides. Confederate Gen. Robert E. Lee moved first. Despite lacking assurance of his own safety, Lee backed off from playing the guerrilla card and made it clear that his fellow Confederate generals should follow his lead. Allowing the Confederacy to continue the war, Lee decided, “would ruin the country, North and South,” Winik says. “It was a step he was unwilling to take. It was a momentous decision.”

At the same time, Lincoln also emphasized conciliation. “Having waged total war, he then said that we must wage a generous peace,” Winik says. “That vision was crucial.” Union Gens. Ulysses S. Grant and William T. Sherman were “touched and moved” by Lincoln’s approach. At Appomattox Court House, “Lee was nervous and afraid of the worst,” Winik says. “Yet Grant treated him with magnanimity, which helped set the tone for the peace to come.”

Of course, it was at this sensitive moment that Lincoln was assassinated. Winik emphasizes that the assassination was actually part of something bigger: a calculated attempt to disable the entire government, with the plotters simultaneously targeting not only Lincoln but also Vice President Andrew Johnson (who escaped injury) and Secretary of State William Seward (who was stabbed but survived). The plan was wickedly clever: At the time, Winik says, the protocols for presidential succession were hazy, and without inspired leadership by both sides, the country could have experienced a meltdown.

“History has imposed this Procrustean inevitability about the end of the Civil War,” Winik says. “When you go back and see it through the eyes and emotions [of the participants], you get a very different picture.” —Louis Jacobson