With the United States starkly divided and with many Americans asking what kind of nation we are, it seems a good moment to look back to November 1863 in Gettysburg, Pennsylvania, when Abraham Lincoln tried to answer the same question. Consecrating a Civil War battlefield where thousands of young men and boys had died four months before, he spoke of a nation “conceived in liberty, and dedicated to the proposition that all men are created equal.” For most Americans since, and for much of the world, those words have at­tained the status of scripture. We draw our sense of collective identity from them. They were, however, not strictly true, and Lincoln knew it.

Five years earlier, he had been more candid. Speaking in Chicago in the summer of 1858, Lincoln noted that when the republic was founded, “we had slavery among us,” and that “we could not get our Constitution unless we permitted” slavery to persist in those parts of the nation where it was already entrenched. “We could not secure the good we did secure,” he said, “if we grasped for more.” The United States, in other words, could not have been created if the eradication of human bondage had been a condition of its creation. Had Lincoln said at Gettysburg that the nation was con­ceived not in liberty but in compromise, the phrase would have been less memorable but more accurate.

The hard truth is that the United States was founded in an act of accommodation between two fundamentally different societies. As one Southern-born antislavery activist wrote, it was a “sad satire to call [the] States ‘United,’” because in one-half of the country slavery was basic to its way of life, while in the other it was fading or already gone. The Founding Fathers tried to stitch these two nations together with no idea how long the stitching would hold.

There were many reasons why this composite nation unraveled in the mid-19th century—but one in particular exposed the idea of the “United” States as a lie. This was the fact that even before the founding, enslaved people repeatedly risked their lives to flee their mas­ters in search of freedom. The Founding Fathers knew the problem firsthand. Many of them were slaveholders themselves, including Thomas Jefferson and George Washington, whose own slaves periodically ran away. And so, in Article IV, Section 2, Clause 3 of the Constitution, which came to be known as the Fugitive Slave Clause, they tried to solve the problem. That clause declared that “no person held to service or labor in one state” could escape from coerced labor by fleeing from a state where slavery was legal to a state where it was illegal.

The constitutional principle was clear, but it proved to be unenforceable. Over the first half of the 19th century, as enslaved men and women ran from slavery to freedom, the federal government remained too weak to do much to stop them. By the second quarter of the century, some of the fugitives—the most famous was Frederick Douglass—were telling their stories with the help of white abolitionist editors in speeches and memoirs that ripped open the screen behind which America tried to conceal the reality that a nation putatively based on the principle of human equal­ity was actually a prison house in which millions of Americans had virtu­ally no rights at all. By awakening Northerners to this fact, and by enraging Southerners who demanded the return of their “absconded” property, they pushed the nation toward confronting the truth that America was really two nations, not one. ...