From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle pp. 239-240:
The well-organized and well-mounted Crows had reigned over a sizable domain that disguised their relatively small population of about five thousand. The upshot was that Crows had placed relatively little pressure on the bison, which remained so numerous that they considered hunting a routine that required neither great numbers nor elaborate planning. Much of that animal bounty now belonged to Lakotas whose expansion had secured them five fertile river valleys—the Tongue, Rosebud, Little Bighorn, Bighorn, and Yellowstone—each filled with bison and enveloped by lush grass. When women and children joined men in the West, bringing in tipis, belongings, and horses, these valleys became home. It was there that Pretty Owl, Red Cloud, Snow-on-Her, Red Woman, Sitting Bull, and many others raised their families, securing their world for generations to come.
The Lakota expansion into the Crow country violated the Horse Creek Treaty, but the government did next to nothing to quell it. Lakotas, after all, were pushing the Crows high up the northern Rockies where that “insolent” tribe posed no threat to America’s overland arteries and hegemonic pretensions. The same applied to the Eastern Shoshones, Assiniboines, and even Mormons whom Lakotas began raiding as the Crows retreated. Soon Lakotas were ranging west of the Bighorns and north of the Yellowstone.
But elsewhere Lakota policies were a direct affront to the United States. In 1853 Cheyennes carried “the pipe around to invite other tribes to war against the Pawnees,” and many Lakotas responded to the call. They killed more than one hundred. Lakotas also raided plains villages along the Missouri and south of the Platte, taking horses and captives and corn and trying to monopolize access to the bison. As if to assert their prerogative to deal with their Native neighbors as they saw best, in the spring and summer of 1860 Lakotas and their allies attacked the Pawnee reservation on the Loup eight times, each strike a blatant violation of the treaty that granted them annuities on the condition of keeping peace. The United States allowed Lakotas to stage raids, wage wars, and expand their empire in its midst without taking effective action to stop them. This passivity is one of the great puzzles in the history of U.S.-Indian relations.
Gold, once again, was at the heart of things. In 1858 rich veins had been discovered in the Rocky Mountain foothills in western Kansas Territory. A hundred thousand “fifty-niners” rushed in along the Solomon, Smoky Hill, and Arkansas Rivers, heading toward Pikes Peak where their shared dreams converged. About half of them returned home the same year, crossing the plains back east. The two-way mass migration left behind wrecked river valleys with pulverized banks almost devoid of game, grass, and timber. Those valleys belonged to Cheyennes and Arapahos, their sanctity guarded by the proudly militant Cheyenne Dog Soldiers. As tensions mounted along the trails, the army sent in soldiers to keep order. The continuing flow of gold through the central plains became a priority for the federal government, and Lakotas slipped into a liberating blind spot. The already limited military oversight shrunk to almost nothing.
The blind spot only grew when the United States descended into civil war. In 1860, pressured by federal agents, moderate Cheyennes and Arapahos accepted a reservation on the Arkansas River, signing away most of their lands. The Dog Soldiers declared the treaty unlawful and began killing anyone—Indians, ranchers, Union freighters—who competed for diminishing resources. Terrified that the Confederates in New Mexico might join with the Dog Soldiers to invade the Colorado goldfields, the Union built a frontier army out of volunteer regiments and assigned it to punish militants across the central plains. Chasing the ultra-mobile nomad warriors absorbed the bulk of the resources the Union could make available in the West. Every raid, fight, and flight the Dog Soldiers staged worked directly to Lakotas’ benefit by keeping the Union preoccupied. Lakotas had a virtual free rein to arrange the world as they saw fit. It was not a coincidence that they were raiding all over the place and enjoying best hunts for a long time during the early years of the Civil War.