Fort Madison in SE Iowa was built to control trade and pacify Native Americans in the Upper Mississippi River region.

During their famous expedition up the Missouri River in seeking the Northwest Passage to the Pacific Ocean, Americans Meriwether Lewis and William Clark noted this spot in June 1804, as they camped for the night just across the river: high commanding position, more than 70 feet above high-water mark, and overlooking the river, which is here but of little depth...[5]In the same year Pierre Chouteau, part of the Chouteau fur trading family and an agent for the Osage, took Osage chiefs to Washington, DC to meet President Thomas Jefferson who promised to build them a trading post.

Previously Jefferson promoted his plan of expanding Federal trading posts on the frontier as means to remove the harmful influence of individual merchants by "undersell[ing] private traders" to make them withdraw from borderlands and "earn the good will of the Indians".

The specific terms of the deal noted:[7] The United States being anxious to promote peace, friendship and intercourse with the Osage tribes, to afford them every assistance in their power, and to protect them from the insults and injuries of other tribes of Indians, situated near the settlements of the white people, have thought proper to build a fort on the right bank of the Missouri, a few miles above the Fire Prairie, and do agree to garrison the same with as many regular troops as the President of the United States may, from time to time, deem necessary for the protection of all orderly, friendly and well disposed Indians of the Great and Little Osage nations, who reside at this place, and who do strictly conform to, and pursue the counsels or admonitions of the President of the United States through his subordinate officers.In exchange for access to the trading post, the attending Osage agreed to cede all of their lands east of the fort in Louisiana Territory to the US.

Sacagawea and her husband, Toussaint Charbonneau, who had accompanied the Lewis and Clark Expedition, also stayed at the fort on their way back north to Dakota Territory after time in St. Louis.

[9] Fort Osage was for many years a productive trading location, with the first Factor George C. Sibley reporting prosperous trade with the Osage due to goods being sold "at prices less than half what the traders extort from them..."[10] The end of the War of 1812 and the Adams–Onís Treaty removed the threat of Spanish or British-backed Indigenous campaigns against the United States throughout the Louisiana Purchase.

Fort Osage formally was closed in 1822, but remained a landmark on the Santa Fe Trail and a transit point for supplies going north.