By Nicholas Lemann

“On Easter Sunday, when the Christian world was chanting anthems in commemoration of the resurrection of the world’s Redeemer, when from every sanctuary the gospel of love and peace was proclaimed, it was then that angels veiled their faces, and devils howled at the bloody and revolting scenes that were enacted on the banks of the Red River.”

That resonant bit of rhetoric comes from a speech delivered by the leading African-American correspondent of the Civil War period, T. Morris Chester, in a speech in New Orleans. The horrifying event he described was the Colfax massacre, which took place in a small town in Louisiana 136 years ago, on April 13, 1873.

Although it has received a flurry of attention in the last few years in several books, the Colfax massacre may still count as the most important moment in African-American history that even people interested in African-American history may never have heard of.

To understand Colfax requires setting the context, which, like the event itself, is undeservedly obscure. After the triumph of “Congressional” or “Radical” Reconstruction in the late 1860s, the principal goals that the NAACP Legal Defense Fund (LDF), and the larger civil rights movement, pursued for so many decades in the twentieth century were largely, though temporarily, achieved.

Black people in the South had civil rights and voting rights. In one of the most impressive feats of political organizing in history, they went from chattel slavery to very high voter turnout in less than a decade. As a result, there were many African-American public officials in the South and substantial government spending on causes important to blacks, chiefly public education.

There was never a very strong white consensus behind these policies, even in the North-Ohio voted against ratification of the Fifteenth Amendment, for example-and in the early days of President Ulysses S. Grant’s second term, what consensus there was began to unravel dramatically.

During Reconstruction no less than in the 1950s and ‘60s, civil rights depended not nearly as much on what laws were on the books as on how seriously they were enforced. In the case of voting rights during Reconstruction, the constant presence of federal troops was required to make them meaningful.

White public opinion in the North was becoming less supportive of federal enforcement of rights for blacks in the South. This made President Grant more hesitant in his deployment of troops, and that, in turn emboldened white terrorist groups in the South whose aim was to use violence to regain political power.

I use the word “terrorist” advisedly-it was widely used at the time. We tend to associate white-on-black vigilante violence in the South with the Ku Klux Klan and with individual local incidents leading to lynchings. In the mid-1870s, though, the Klan, while retaining its sexual and violent preoccupations, morphed into more organized and strategic statewide organizations (Louisiana’s was called the White League) that functioned, as one historian memorably put it, as the terrorist wing of the Democratic Party.

These organizations staged an open challenge to Reconstruction, which began roughly at Colfax in 1873 and ended with the presidential election of 1876-and they won. The federal government stopped enforcing the Fourteenth and Fifteenth Amendments in the South, and from there it was a direct route to the Jim Crow regime that the legislatures of the former Confederate states created in the 1890s, and that lasted until the civil rights era.

Colfax was a tiny hamlet on the Red River, in rich-soiled plantation country just north of Alexandria, Louisiana. It had black (and that meant Republican) political officials. On April 1, 1873, a white armed force appeared, meaning to take political power by force. Over the next few days, blacks from Colfax and the surrounding area gathered around the tiny county courthouse, to defend it and themselves from the white militia.

On Easter Sunday, the whites attacked, and as the day wore on, they crossed the line from military victory to killing frenzy. Dozens of blacks were murdered, at Colfax and in a subsequent manhunt in the countryside. Many were crudely hacked to pieces. Historian Eric Foner called the Colfax massacre the bloodiest single incident of the Reconstruction period, which is saying a lot.

The leaders of the Colfax massacre were rounded up by federal troops and brought to trial- but none of them was convicted of a crime. This emboldened the white terrorists, who became immediately much more active, especially in Louisiana and Mississippi.

Even worse, for the long term, the prosecution that followed the Colfax massacre became the basis of a landmark 1875 Supreme Court decision, United States v. Cruikshank, in which the Court ruled that the states, not the federal government, were the only appropriate enforcers of the Fourteenth and Fifteenth Amendments. The Cruikshank decision was one of the main nails in the coffin of Reconstruction.

It is not very uplifting to remember the Colfax massacre, but it is necessary. It should remind us of how difficult-nearly impossible-it was to achieve some of the civil rights that most Americans now take for granted, and of how evanescent those rights proved to be when they were achieved in the first place.

Nicholas Lemann, Dean of the Columbia University Graduate School of Journalism, is a prize-winning author whose books include The Promised Land: The Great Black Migration and How It Changed America; The Big Test: The Secret History of the American Meritocracy; and most recently, Redemption: The Last Battle of the Civil War.