Three hours from Denver, in the bleak, scrubby flatlands of eastern Colorado, is the Sand Creek Massacre National Historic Site. Here, in November 1864, a volunteer militia commanded by Col. John Chivington descended on a winter camp of Cheyenne and Arapaho Indians. Some said 70 died; others more than twice that number. No one took the trouble to do a body count. It’s now conventional to refer to slavery as America’s original sin; Sand Creek is a reminder that there were two. Left unsaid is how closely the two were intertwined.

Many of the mass killings during the Indian Wars are now memorialized in National Battlefields, National Historic Sites, and National Monuments. The way in which they are recorded and interpreted is an attempt to express some rough national consensus about their meaning, something only the federal government can ultimately hope to articulate. It will no doubt happen one day with monuments to the Confederacy and slavery, even if it takes more decades of bitter rancor and sporadic violence.

Sometimes the consensus over the Indian Wars has been near-impossible to reach, even a century and half later. When the Little Bighorn Battlefield National Monument was created 20 years ago, there was furious opposition to the idea that it should honor the Indian dead as well as the 263 cavalrymen who died under Gen. George Armstrong Custer. In the end, there was an awkward compromise: in addition to the rows of American tombstones, a skeletal sculpture of three Indian warriors on horseback. Yet the Sioux and Cheyenne still bitterly resent the very name of the monument. Was it the Battle of the Little Bighorn or, as they call it, the Battle of the Greasy Grass?

At Sand Creek, there is no such dispute. The visitor center makes no attempt to say that there were “very fine people on both sides.” Sand Creek was the My Lai of the Plains Wars. The chief of the defenseless camp was friendly to the whites. Chivington was a fanatical preacher and a murderous psychopath. The brutality was extreme. Chivington’s men, many of whom were falling-off-their-horses drunk, excised the genitals of Arapaho women and displayed them later on the stage of a Denver theater to wild applause.

Yet less than ten miles away from the battlefield on Route 96 is a small, unincorporated community, largely abandoned but with a few modern homes and doublewide trailers. The town has never changed its name, which is Chivington.

In other words, no matter how unequivocal the verdict of the federal government on Sand Creek, the memory of Colorado itself is another matter. While our attention has been riveted on the states of the Old South, Colorado too has been caught up in the turmoil over Confederate monuments because it has six of them, including two on public land. Governor John Hickenlooper has promised to “continue the conversation” about what should be done about them. More surprising, however, is the debate that has simmered for several years now in Colorado over a monument that shows how intimately the Civil War and the Indian Wars were connected.

The Colorado Civil War Monument, erected in 1909, stands right outside the state capitol in Denver. It is not a tribute to the Confederacy. The statue atop the plinth honors an anonymous soldier—of the Union. Engraved on it are the names of Union military commanders and a list of the Civil War battles that were fought in what was then Colorado Territory. The roster of heroes includes Col. John Chivington, and Sand Creek is named as one of the battles—something it never was.

And yet, in the minds of Coloradans, in the 1860s and for many decades after, that’s exactly what it was. Our tortured national struggle over the enduring legacy of the Civil War has all been about the South. Absent from the debate has been any mention of the Civil War in the West, and the fact that violent white supremacism was not the monopoly of the Confederacy.

Sand Creek did not just happen at the same time as the Civil War. It, as well as many other mass killings, was seen in the West (and by military commanders in the East) as a function of that war, an auxiliary battlefront. If Chivington’s men were unusually cruel, it was because the intense pressure on the Union Army east of the Mississippi made it necessary to enlist poorly trained and ill-disciplined volunteers in the West. The roots run much deeper, however, going back to the gold rush in the mountain states. This led, as it had previously in California, to the violent dispossession of the tribes who had the misfortune to live in the area of the gold strikes—Cheyenne, Arapaho, Ute, Shoshone, Nez Perce—and the abrogation of the treaties that had been made with them. But more than that, western gold was desperately needed for the flagging Union war effort.

In the summer of 1862, Robert E. Lee and Stonewall Jackson were in their glory. Between May and August of that year, Jackson overwhelmed superior Union forces in the Shenandoah Valley, while Lee drove back “Little Mac” McClellan’s assault on Richmond and then routed the North at Manassas. But on July 28, in the midst of these disasters, came the first gold strike in Montana Territory. The timing could not have been more opportune. Millions of dollars in nuggets and dust began to flow eastward into the Union coffers, playing a significant part in strengthening the North as it turned the tide of the war.

The western mines were not just a source of support for the Union cause; the Civil War was also fought out there by proxy. Although the Montana gold camps were critical to the Union war effort, most of the miners were militant “Seceshes,” and they engaged in a fierce battle over names and symbols of the new territory. Southerners called their new settlements Fort Sumter, Dixie, Confederate Gulch, and Varina—to honor the wife of Jefferson Davis—until a politic judge offered the compromise name of Virginia City. “Seceshes” burned the Stars and Stripes; men danced in the streets to celebrate the assassination of Lincoln and posted notices that read, “Old Abe has gone to Hell! Hurrah for Jeff Davis!”

One thing united the warring factions, however, and that was the doctrine of white supremacy. Many leaders of the Northern settlers were highly educated men, graduates of Harvard and Yale, and familiar with the writings of early social anthropologists like Lewis Henry Morgan, whose theory of the evolution of human society foresaw the inevitable triumph of “civilization” over “savagery” and “barbarism.” For some supporters of the Union, this was tinged with pangs of conscience about the unfortunate necessity of doing away with the noble Red Man; at the other extreme, it led to the racist savagery of John Chivington.

Once the North had triumphed, the leaders of the Union Army turned their attention fully to the West. General “Little Phil” Sheridan, the hero of Appomattox, was given overall command of the vast area between the Mississippi and the Rockies, where he imported the doctrine of total war against the homes, possessions, and food sources of the civilian population that he had pioneered in “The Burning” of the Shenandoah Valley in 1864, adding the new element of mass killing, regardless of age or gender. Today his name is honored in the city of Sheridan, Wyoming: a tribute either to the conqueror of slavery in the East or to the architect of genocide in the West. It depends on whom you ask.

Sheridan’s local commanders and civilian admirers are similarly memorialized. Each year, visitors to Yellowstone National Park snap selfies with the great herds of buffalo in Hayden Valley, which honors the geologist-explorer who declared that his work would be easier if “the government sends troops up here and wipes out two or three hundred [Indians].” One of the officers who obliged, cavalry lieutenant Gustavus Cheyney Doane, led the massacre of at least 173 Blackfeet on the Marias River in Montana, most of them women and children and many suffering from smallpox; the perfect cone of Mount Doane is one of Yellowstone’s most striking peaks. Fly fishermen ply the pools and riffles of the Gibbon River, named for a colonel who commanded a division with Sheridan at Appomattox and then wiped out between 70 and 90 Nez Perce Indians who had been driven from their lands by the gold rush in Idaho.

It’s natural, of course, that our main focus should remain on the South. But this isn’t only because that was the home of the Confederacy. It’s also because—mercifully if imperfectly, and at great continuing cost—African-Americans have clawed back enough political standing to force their voices into the public debate. The surviving American Indians in the West have no such voice; their suppression was too complete. So they languish in the squalor of the rez, out of sight and out of mind, absent some rare spasm of resistance like last winter’s Standing Rock protests against the Dakota Access Pipeline. But until we understand the role that they and their killers—loyal Union men to a fault—played in the Civil War, we will never have a full understanding of all the terrible dimensions of that conflict.