“WE HOLD THESE truths to be sacred and undeniable,” Thomas Jefferson wrote in his first draft of the Declaration of Independence (the composition of which was regarded as something of a minor administrative chore, as noted by the historian Joseph Ellis). He showed the draft to John Adams and Benjamin Franklin, who suggested “self-evident” instead — perhaps a polite way of saying to the British crown: “as any fool can plainly see.” Although Jefferson (uncharacteristically) accepted the change without complaint, the “truths” that all men are created equal and endowed with certain unalienable rights were not self-evident to everyone, even in the American colonies, where a sizable part of the population was loyal to Great Britain and a great many more simply wanted the revolutionary turmoil to end so things could get back to normal.

The obvious contradiction of the Declaration’s fine sentiments — the institution of slavery — had to be swept under the rug in 1776 lest southerners withdraw from the fragile and sometimes fractious coalition that was forming behind the idea of independence. “Nevertheless,” writes Mr. Ellis, “what Jefferson had done, albeit inadvertently, was to smuggle the radical implications of the American Revolution into the founding document, planting the seeds that would grow into the expanding liberal mandate for individual rights that eventually . . . ended slavery, made women’s suffrage inevitable, and sanctioned the civil rights of all racial minorities.” And some other minorities as well, we could add.

These rights and “truths,” the very idea of one self-governing nation, are grand concepts, abstract and elevated. One hundred and fifty years ago today, though, a field in Pennsylvania bore witness to the grim, flesh-and-blood realities of securing and defending them: thousands of men lying dead and wounded, pelted by a torrential rain after three days of slaughter.

This was a campaign that the Confederacy hoped would so discourage the North as to bring a settlement of the war. Shortly before the battle was joined, there were a number of incidents that made clear just how great a betrayal of the country’s past and future that would have been: The invading Confederate forces passing through small Pennsylvania towns indiscriminately rounded up black people wherever they found them, with little regard for whether they were free or escaped slaves. They were, after all, valuable property. Writes the historian Allen Guelzo: “As one farmer was told by Confederates who were escorting ‘four wagonloads of women & children between Chambersburg & the Maryland line,’ even the children ‘will bring something.’ ”

Gettysburg’s sesquicentennial todayis a noteworthy reminder that such things once happened on U.S. soil, and were accepted by many. Selfish interests, faction, prejudice and the will to domination will always blind a certain number of people to the “self-evident” truths of the Declaration. All the more reason to keep in mind that they are also, as the unedited Jefferson would have it, “sacred and undeniable.”