From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle pp. 155-156:
In the summer of 1815, seven months after the Treaty of Ghent, U.S. officials invited the western Indians to a council at Portage des Sioux just north of St. Louis. Two thousand Indians showed up, and Americans made treaties with Lakotas, Mdewakantons, Wahpekutes, Sissetons, Wahpetons, and Yanktons. The pithy compacts pardoned past aggressions and brought the Sioux under the protection of the United States. The Americans understood the last article as a corroboration of U.S. jurisdiction over the Sioux, but the seventy-two Sioux delegates who touched a pen probably understood it as a confirmation of the prewar status quo whereby Americans traded with them without dictating to them. When U.S. soldiers began building an unauthorized fort at Prairie du Chien a year later, Mdewakanton chiefs approached British agents in the upper Great Lakes, pleading for help in preventing their “final extinction.” It was only when the agents refused them that the chiefs realized that they would have to face the United States without British counterweight. Their fall from power was shockingly fast.
Left alone to face the Americans, Dakotas were soon reeling. In 1818 Benjamin O’Fallon, a newly appointed U.S. agent for the Sioux and William Clark’s nephew, led two heavily armed keelboats up the Mississippi and Minnesota to stop Canadian incursions and establish American authority in the region—an enterprise that echoed his uncle’s famous expedition fourteen years earlier. O’Fallon found the Sioux divided and quarrelling over trade. At a Mdewakanton village, chief Shakopee, “ferocious and savage,” complained that his warriors lacked guns and could not contain their Chippewa enemies. O’Fallon urged them to “be always last in war” and place their faith in “the Great Spirit.”
Cut off from the British trade and political support, the Mdewakantons accepted O’Fallon’s gifts—a little whiskey and some goods—and demands. Within a year the U.S. Army started planning a military fort at the Mississippi-Minnesota junction on lands Pike had purchased fourteen years earlier.
O’Fallon had succeeded where Lewis and Clark had failed. While the Corps of Discovery had inadvertently prompted Lakotas to strengthen their hold of the Missouri and its peoples, O’Fallon had extorted from Dakotas a tacit acceptance of a military fort on their lands. Fort Snelling was in operation by 1820 and was soon accompanied by St. Peter’s Indian agency. The complex marked the beginning of a growing American presence in Dakota lives. It became a hub for the growing fur trade, which soon cut into the region’s animal populations, creating food shortages and entangling Dakotas into chronic wars over hunting privileges with Chippewas and Ho-Chunks [aka Winnebago]. Indian agents tried to mediate, but they lacked the know-how to be effective. Things came to head in 1827 when Mdewakantons and Wahpetons killed two Chippewas in a council at Fort Snelling under a U.S. flag. The fort commander, Josiah Snelling, imprisoned several Dakota warriors and demanded the culprits be turned over to him in return for their release. He was given a few men whom he handed over to Chippewas. Chippewas let them run before shooting them down. Then they scalped them in front of the shocked American officials.
A century earlier, less than fifty miles downriver from Fort Snelling, the French had built Fort Beauharnois to serve Dakotas. That trading fort had been the focal point of a deep Dakota-French accommodation that stabilized the upper Mississippi Valley, fueled the expansion of the fur trade west of the Great Lakes, and made the Dakotas the dominant power in the interior. Now the region’s dominant fort was a military establishment that heralded U.S. sovereignty over the Mississippi Valley, monitored the Dakotas, and staged U.S.-sponsored public executions of Dakota people.