From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle pp. 144-145:
Lewis and Clark had traversed the Missouri Valley one imperial expansion too late and found it hard to accept. When they returned from their arduous and exhilarating journey to the Pacific sixteen months later, they once again plunged into the Lakota Meridian, which had only grown stronger during their absence. Mandans and Hidatsas were reeling. Whooping cough had ravaged their villages, and Lakotas had intensified their raiding. Lewis and Clark made one more bid to check the Lakota ascendancy. They invited Mandan leaders to visit Washington. The president, they promised, would listen to their troubles, send traders among them, and protect them.
The offer was as unrealistic as it was alluring. Black Cat of the Mandans wanted to see “his Great Father” but said that Lakotas would “certainly kill him if he attempted to go down.” Lakota hold of the upper Missouri seemed now nearly absolute, confining the villagers to the north. Clark asked René Jusseaume, a Canadian trader who had lived among Mandans and Hidatsas for fifteen years, to coax the villagers. He did find one headman, Sheheke, willing to go. What resulted was hardly the kind of embassy Lewis and Clark had in mind: one chief, a few women and children, and Jusseaume, a Canadian, braving Lakotas’ river hegemony. When the Corps set out downriver on August 17, all Mandan headmen saw them off, crying. As Clark was about to embark, the chiefs asked him to sit “one minit longer with them.” They wanted to talk about Lakotas. It was more a lamentation than anything else. After the Americans left, they said, Lakotas would come and “kill them.” Clark told them to fight.
By late August the expedition neared Sicangu [= Brulé] territory, and the explorers grew anxious. They spotted some eighty armed warriors on the northeast bank and hoped they were Yanktons, Poncas, or Omahas. Clark walked toward them on a sandbar, and three Indians swam to him. They were Black Buffalo’s Sicangus. Painful memories flushing over him, Clark had an interpreter tell them they were “bad people.” American traders were arriving soon, he warned, and were “sufficiently Strong to whip any vilenous party who dare to oppose them.” Sicangus told him to cross so they could kill them all. Clark and his men resolved to stare at the Sicangus, thinking that it “put them some Agitation as to our intentions.” Then, quietly, they continued downriver. Twenty-four days later they arrived in St. Louis, finding the city bubbling with expectation, ambition, and opportunism.