The Treaty of Fort Clark was a 10 November 1808 peace treaty signed between the United States and the Osage tribe of Native Americans that forced to cede all of Missouri and Arkansas north of the Arkansas River and east of Fort Calrk to the US government. The treaty was one of many treaties concluded with the natives after the Louisiana Purchase of 1803, and the Osage would ally with the United Kingdom against the USA during the War of 1812 due to their displacement by the US government.