From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle pp. 80-81:
The booming trade in the Mississippi Valley drove Lakotas deeper into the prairies in search of castor. Impatient with rival hunters from the Missouri River villages, they fought more intensely and widely: almost every midcentury Sicangu [= Brûlé] winter count records a clash with some enemy group or another. Sicangus now had guns, enough to seize the prairies and their beaver streams all the way to the turbulent, meandering Mníšoše [Missouri R.]. It was sometime during this westward thrust that they launched their first mounted raid. “Went on the warpath on horseback to camp of enemy,” the caption for the 1757–58 count reads, “but killed nothing.” In spite of the underwhelming outcome, it was a transcending event. Sicangus had accumulated enough know-how to wield a lance on a beast galloping at twenty to thirty miles per hour, enough animals to put an entire war party on horseback, and enough confidence to launch an attack. Sicangus edged westward in a close partnership with Oglalas, who probably began experimenting with mounted warfare around the same time.
Sicangus and Oglalas pushed to the James River and beyond, entering the Coteau du Missouri, a poorly drained plateau that flanks the eastern side of the Missouri River. The Coteau was an inferior trapping and hunting country, and Sicangus and Oglalas moved rapidly across it, facing little resistance. They touched the Mníšoše amid several Arikara villages that housed thousands of people. For Arikaras the Missouri was tswaarúxti, Holy Water, along which all of their history had happened. They lived in some thirty villages on its banks and were determined to keep the newcomers out. Sicangus and Oglalas found a relatively safe niche in an eastward protruding meander that would be later known as the Big Bend of the Missouri. Stretching out for thirty miles in near full circle, the bend enveloped a fertile, grass-covered island that teemed with bison, elk, and other game—a superb base from where Sicangus and Oglalas could raid Arikaras for food, horses, and captives and dominate the prairies to the east.