On October 20, 1803, the Senate approved a treaty between the United States and the French Republic. This treaty was no small affair. It gave the United States an astonishing 828,000 square miles, doubling the size of the original United States. Since then, people have made much of what seemed a ridiculously low price for the purchase—$15 million, which amounted to about $.04 an acre. But there was a cost to the Louisiana Purchase that is rarely reckoned: with it, the United States bought a century of warfare over contested land.

The region that lay within Louisiana—now comprising the states of South Dakota, Nebraska, Kansas, Oklahoma, Arkansas, Missouri, and Iowa, and most of North Dakota, Montana, Wyoming, Colorado, Minnesota (and, of course, modern-day Louisiana)—was some of the richest land on the continent. It had grasslands where bison thundered, and mountains teeming with elk, moose, deer, waterfowl, and beaver. It had mineral deposits of gold, silver, lead, and copper.

But this region had an extensive population of Indians who were not keen on interlopers. Unlike native peoples east of the Mississippi, the Plains tribes had adopted the horse after the Spanish had brought this European animal to American shores. Once horsed, Plains Indians became formidable warriors. Famously, Comanche fighters could ride into a battle with one leg slung over their pony’s neck, firing from beneath the animal’s head as they kept the pony between the soldiers and themselves. They shot arrows with enough force to go through a bison, and they could shoot them fast enough to keep one in the air at all times. Not just the Comanche, but the Kiowa, Cheyenne, and Lakota, as well as other tribes, could hold their own against the nineteenth-century American military.

They did so, successfully, until the Civil War, largely because few emigrants were eager to settle in the Great Plains, a region known in those days as the Great American Desert. There were too few trappers and miners venturing in the territory to involve the military very deeply there. But during the Civil War, new mines in Montana attracted eager young men at the same time that Congress had chartered a transcontinental railroad to take farmers to the Plains. By the end of the war, miners, farmers, and railroad crews were invading Kiowa, Comanche, and Lakota land.

So dire was the warfare on the Plains that American leaders sent General William Tecumseh Sherman, fresh from his victory over the Confederacy, to subdue the Indian fighters. Furious at those people he believed were stopping the triumphant spread of the American system, Sherman insisted that the Indians must be corralled to the north and south of the main transcontinental railroad line. In 1867, the Treaty of Medicine Lodge established southern tribes on a reservation in what is now Oklahoma; in 1868, the Treaty of Fort Laramie pushed the northern tribes to the Dakotas and Montana.

But the treaties stopped the fighting only temporarily. Railroad crews continued to hammer their way onto Indian reservations. Then, in 1875, General George A. Custer led an expedition that discovered gold in the Black Hills, and miners rushed to grab Lakota land. When army officers demanded that Indians stop resisting the interlopers, leaders like Sitting Bull and Crazy Horse gathered around them the largest group of warriors ever assembled on the Plains. In the June 1876 Battle of Little Big Horn, in a battle fought on land guaranteed to them by treaty, they beat back the U.S. Army. Their victory was temporary, as troops hunted down the Indians, forcing all but a few bands to surrender by 1878.

And yet, the fighting still was not over. The bad blood left over from the 1870s helped in 1890 to spark official concerns that Northern Cheyenne and Lakota would murder settlers in Montana, Nebraska, and the new states of South Dakota and North Dakota. In December of that year, soldiers sent to round up potential warriors massacred 250 Lakota and Cheyenne at Wounded Knee Creek in South Dakota.

In 1903, a Kiowa, Lone Wolf, challenged the legality of the U.S. acquisition of his tribal lands. In Lone Wolf v. Hitchcock, the Supreme Court denied Lone Wolf’s plea. “It is to be presumed that in this matter the United States would be governed by such considerations of justice as would control a Christian people in their treatment of an ignorant and dependent race,” the Court wrote. “Be that as it may, the propriety or justice of their action towards the Indians with respect to their lands is a question of governmental policy, and is not a matter open to discussion in a controversy between third parties….”

From 1803 to 1903, for a full century, the ownership of the Louisiana Territory remained open to debate, something senators could not have imagined when they voted “aye” on October 20, 1803.