The reason the South fought the American Civil War has been contested ever since the Confederacy surrendered in 1865. An odd turn of events, considering that when 11 Southern states seceded from the Union at the war’s outset, they were very clear about why they were doing it.

In declaration after declaration, Confederate states explicitly said that they had seceded in order to preserve slavery.

South Carolina, the first to secede, cited “an increasing hostility on the part of the non-slaveholding States to the institution of slavery” in its declaration of secession. Mississippi’s declaration argued “There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union.”

It was only after the war that many former Confederates changed course, creating an alternative narrative that historians refer to as the “Lost Cause.”

“It began right at the end of the Civil War as Southerners tried to explain their own defeat to themselves,” says David W. Blight, an American history professor at Yale and author of Race and Reunion: The Civil War in American Memory. Writers, journalists, and former soldiers began “to fashion this series of ideas, one of which was their belief that they were never truly defeated on the battlefield; that they were only overwhelmed.”

They also argued, in direct contradiction to their secession statements, that the war was never about slavery.

Lost Causers argued “they had only fought for state sovereignty, states’ rights, national independence,” Blight says. “They also fashioned a set of ideas and arguments that they were fighting to hold back the massive industrialization of America, they were trying to preserve rural agrarian civilization.”

In addition, they gave the cause a hero. When Robert E. Lee died five years after the war ended, many of his former officers “created a kind of a Lee legend and a Lee cult,” he says. It promoted the “idea that Robert E. Lee was the ultimate Christian soldier,” who fought to preserve his home state rather than the institution of slavery—which is false.

“Make no mistake, Lee fought for the Confederacy, and he knew that the Confederacy existed to preserve slavery—there is no question about that,” Blight says.

To further bolster their hero, the Lost Causers also gave Lee a villain: Former Confederate General James Longstreet, who was already a “scalawag” for joining the northern Republican party and deploying black and white officers to defend New Orleans against the militant White League during Reconstruction. According to this new hero-villain narrative, Lee had lost the Battle of Gettysburg because Longstreet betrayed him. (Blight says this “does not hold up historically”).

Over time, the narrative morphed as more people—including former President of the Confederate States of America Jefferson Davis—wrote about and memorialized the war.

“By the 1890s, the Lost Cause arguments had become really a racial ideology, they had become a set of arguments for white supremacy,” he says. The idea that slavery had been a gentle institution that benefitted both masters and slaves, and that freedmen could not handle their emancipation, was a foundation upon which Jim Crow laws were built.

And as the South began to beat back Reconstruction policies with these Jim Crow laws, the narrative actually stopped being about loss.

It became “a victory narrative,” Blight says. “And the victory they’re telling is the victory over Reconstruction: That they had defeated the North’s effort to reconstruct the South, that they had defeated black rights and black suffrage.”

Confederate veterans and Southern organizations worked to make sure that school textbooks portrayed the Confederacy’s goal as righteous and Lee as a noble hero, effectively changing the way that the war and its causes were understood. This strategy worked so well that it influences education today. In recent years, Texas has adopted school textbooks and lesson plans that incorrectly teach students that slavery was not a major cause of the war.

“It’s endlessly necessary in this country to keep explaining the Civil War,” Blight says. “There’s a great distance between public memory and the scholarly history that historians write. And we just have to keep trying to make that distance shorter.”