When William Clark and Meriwether Lewis’ famous expedition reached the Great Plains in 1806, the crew couldn’t believe their eyes. There, ranging across the prairies in groups that moved up to 30 miles per hour, were gigantic herds of buffalo so large they waylaid the travelers for hours on end. “The moving multitude…darkened the whole plains,” they wrote.

Seventy years later, buffalo—and the Native American tribes the explorers encountered—would be almost eliminated on the Great Plains thanks to another revered American, William Tecumseh Sherman.

The general is best known for his for his bloody march across Georgia, stealing livestock and intimidating civilians in one of the most famous campaigns of the Civil War. But after the war, he didn’t retire. Instead, he took his scorched-earth tactics to another war—one against Native Americans. Sherman’s campaign led not just to the extermination and relocation of thousands of indigenous people, but the annihilation of nearly all of the United States’ wild buffalo.

The Civil War might not seem related to the fate of buffalo or Native Americans, but in reality they were closely intertwined. Almost as soon as the Civil War ended, the United States turned its attention westward. A country that couldn’t agree on slavery rallied around the idea that American should push west.

There was just one problem: The land was already settled by Native Americans, many of whom put up armed resistance to American incursions. “Northerners and Southerners agreed on little at the time except that the Army should pacify Western tribes,”write historians Boyd Cothran and Ari Kelman.

In the eyes of the government and the public, there was one man perfectly suited for the job: Sherman. Now the most senior member of the U.S. Army, Sherman was known for using psychological warfare to bring the South to its knees. By sending Sherman west, officials hoped, the United States could gain even more land and secure space for an ambitious westward expansion.

Buffalo were a critical part of that plan. Sherman’s job was to use the U.S. Army to protect the transcontinental railroad and secure mining interests in territory traditionally owned and settled by Native Americans. The plan was to force Native Americans onto reservations, seize their land and protect the settlers who moved there. In a series of campaigns now known as the western Indian Wars, the military clashed with tribes intent on protecting their lands and their way of life.

The buffalo were a critical part of those traditions. Plains Indians relied on bison for food and housing, and the wild buffalo was seen as a sacred animal. At the time, between 30 and 60 million buffalo are thought to have roamed the plains—and Sherman knew that if the buffalo went, so would Native Americans. “The government realized that as long as this food source [and] key cultural element was there,” anthropologist and Native American studies professor S. Neyooxet Greymorning told Indian Country Today, “it would have difficulty getting Indians onto reservations.”

Destroying the buffalo meant destroying Native Americans, so Sherman homed in on the animals. As long as the buffalo roamed, he wrote to fellow general Philip Sheridan in 1868, Native Americans would follow them. “I think it would be wise to invite all the sportsmen of England and America…this fall for a Grand Buffalo hunt,” he wrote, “and make one grand sweep of them all.”

In response, the U.S. Army threw its support and protection behind civilian hunters, who headed west to slaughter buffalo. Though the U.S. Army itself never conducted an organized hunt, it allowed and encouraged soldiers and civilians to kill huge numbers of animals.

One of them was William “Buffalo Bill” Cody, who got his nickname after killing a reported 4,280 buffalo during an 18-month period. Cody was escorted by the U.S. Army during numerous hunting contests, competing against fellow hunters to bring back the most buffalo tongues. And so-called “hide hunters”—supported by Sherman’s army—killed and skinned tens of thousands of buffalo, creating a new leather alternative and a bustling industry that relied on buffalo skins.

By 1873, buffalo were nearly extinct. “Where there were myriads of buffalo,” wrote U.S. Army colonel Richard Irving Dodge, “there were now myriads of carcasses. The air was foul with a sickening stench, and the vast plain…was a dead, solitary, putrid desert.” Yet Sherman still argued that the United States needed to keep killing buffalo to subjugate Native Americans.

It worked: By the early 20th century, only 325 buffalo remained in the entire country. Historians now attribute nearly three quarters of Native American population decline to westward expansion. Between 1800 and 1890, the Native American population dropped from around 600,000 to just 228,000.

By the time Sherman retired in 1884, he had succeeded in forcing Plains Indians onto reservations. As historian David D. Smits writes, “With the mainstay of their diet gone, the Indians had no choice but to accept a servile fate on a reservation where they could subsist on government handouts.” And in the words of the Sioux leader Sitting Bull, “a cold wind blew across the prairie when the last buffalo fell—a death-wind for my people.”

Ironically, Sherman himself was named after a Native American peacekeeper, Tecumseh. The Shawnee chief formed and led a large confederacy of tribes that fought the United States during the War of 1812. Americans admired Tecumseh’s ability to bring together different tribes and fight with what they saw as noble intentions.

Like Tecumseh, Sherman was a savvy leader—but unlike the chief, he fought not to preserve Native American culture, but to destroy it.