He stood on the outer edge of the sidewalk, hands clasped behind him—handcuffed, perhaps, by the immensity of the moment. He knew the city of Springfield, Illinois, well. But on June 16, 1858, Abraham Lincoln was learning his new place in American politics, and possibly dreading what it now demanded. He was about to deliver a speech accepting the nomination as the Republican Party’s candidate for the U.S. Senate. In it, he planned to diagnose a malignant divide many of his fellow Republicans believed was benign.

The speech was too radical, his friends had told him, as friends had told American revolutionaries a century earlier. Those revolutionaries had pressed on with their cause. Summoning the courage to press on with his, Lincoln may have reminded himself that ignoring the divide would be more radical still.

Delegates stared at Lincoln as they made their way to the Illinois state capitol. Patrons stared at him from the nearby dry-goods store owned by his friend John Williams. Lincoln believed that the slave states were staring at him, too—were eyeing the northern states as well as the western territories. With the passage of the Kansas-Nebraska Act of 1854, the election of the expansionist President James Buchanan in 1856, and the Supreme Court’s Dred Scott decision of 1857, all three branches of the federal government had acquiesced to the march of slaveholders across the nation.

Lincoln left the solitude of the sidewalk. He walked across the square, entered the House of Representatives, and stood before the more than 1,000 delegates of the Illinois Republican State Convention. “A house divided against itself cannot stand,” he declared. “I believe this government cannot endure, permanently half slave and half free … It will become all one thing or all the other.”

The prophetic speech propelled Lincoln onto the national stage. Three years later, as president, he was commanding an army that battled to keep the house standing. His opponent, the Confederate States of America, was fighting to destroy the house and to build a new one safe for slavery. ...