On November 10, 1808, at the Osage Treaty or the Treaty of Fort Clark, the Osage Nation ceded it's territory in Missouri and Arkansas to the US. From the article:
"Osage - Treaties With the United States - Kansapedia
A delegation of 12 Osage chiefs met with President Jefferson in July 1804 in Washington, D. C. Jefferson believed it was necessary to establish relations with the Osage because the United States had a weak presence in Osage territories. Jefferson pushed for peace between the United States and the Osage, an increase in fur trade, and permission for American exploration of the west. Jefferson did not reveal his intention of moving eastern tribes into Osage territories. This led the Osage to falsely believe the United States had only benevolent intentions with them, which influenced later agreements.
The Osage made seven treaties with the United States between 1808 and 1839. The treaty of 1808 was made in St. Louis, establishing Fort Clark (Fort Osage) as a federal trade factory, set regulations on the fur trade, provided a blacksmith at the fort to provide and repair metal trade goods, and had articles to negotiate cultural and territorial disputes. The articles on disputes, however, were not fair to Osage interests. The treaty of 1815 renewed peace agreements between the Osage and the United States in the wake of the War of 1812 and served to alleviate fears of an Osage and British alliance. The treaty of 1818 was a response to Euro-American and eastern tribes’ intrusions into Osage lands. It led to Osage land cessions to the United States, which were used to push the Cherokee farther west.
The treaty of 1822 closed Fort Osage as a federal trade factory and paid the Osage $2,329.40 in merchandise. The first treaty of 1825 led to Osage land cessions in present-day Oklahoma, which the United States used as Cherokee land. William Clark represented the United States at the Council Grove Treaty in 1825. The negotiations took place in present-day Council Grove, Kansas. The Osage agreed to cease raids on the Santa Fe Trail, which allowed the trail to be cleared for pioneers. The Osage living in present-day Oklahoma ceded all territory to the United States and were removed to present-day Kansas by the treaty of 1839. The Osage were paid in trade goods that profited the suppliers, but were of little use to the Osage.
There were three major periods of Osage land cessions to the United States between the Louisiana Purchase and 1839. Three tracts of Osage land were ceded from an area spanning the majority of present-day Missouri and the northeastern quarter of present-day Arkansas in 1808. Osage territories between the White, Arkansas, and Neosho-Grand Rivers in present-day northwestern Arkansas and northeastern Oklahoma were ceded in 1818. Land between the southern border of present-day Kansas and the Canadian River in present-day Oklahoma, the eastern edge of present-day Missouri, and much of the central part of present-day Kansas, south of the Kansas and Smoky Hill Rivers were ceded in sections between 1825 and 1839. This diminished Osage territory to a narrow strip across the southern half of present-day Kansas.
In a period of 31 years, Osage territory had diminished by more than 75 percent. The Osage lands in present-day Kansas were eventually ceded in 1870. The Osage Reservation in present-day Oklahoma was purchased at the end of the 1870s. The Osage were one of the few tribes to purchase their reservation. In a technical sense, the Osage were not removed in the same manner as other tribes. The Osage Reservation was part of Osage territory in Oklahoma that had previously been ceded to the United States and divided among emigrant tribes that had been removed from their home territories. Although the Osage were not arbitrarily moved to their reservation’s location, they were not spared the harmful impact of removal.
Jesuits and Education
The Osage had contact with Jesuits, whom the Osage called “Black Robes,” since some of their earliest contacts with Europeans. Many French men intermarried with Osage women under Osage law, but some of these marriages were performed by Jesuits as early as 1750. Jesuits lived with and established relationships with the Osage in Missouri in the 1820s. The first Protestant-run Osage missions were founded in 1821, with the goal of converting the Osage as the primary objective and educating them as an afterthought. These Protestant missions were a failure and the last Protestant Osage mission closed in 1837. The Osage petitioned President Tyler to use the Osage Education Fund, which was created from the sale of Osage lands in the Treaty of 1825, to bring Catholic missionaries, specifically Jesuits, to the Osage on June 14, 1843.
After fighting obstacles from the United States government, the Jesuits arrived at the Osage Mission in present-day St. Paul, Kansas, on April 28, 1847. The school opened on May 10, 1847. The Jesuits believed that it was a waste of funds to only educate boys, so they opened a girls’ school with help from the Sisters of Loretto from Nerinckx, Kentucky, on October 10, 1847. The Jesuits had a stronger emphasis on educating the Osage than the Protestant missionaries, which was crucial to their success. Although Osage culture was altered in the process, the Jesuits helped the Osage survive the challenges of the following two decades."