Fort Osage was an early 19th-century factory trading post run by the United States Government in western Missouri on the American frontier; it was located in present-day Sibley, Missouri. The Treaty of Fort Clark, signed with certain members of the Osage Nation in 1808, called for the United States to establish Fort Osage as a trading post and to protec…Fort Osage was an early 19th-century factory trading post run by the United States Government in western Missouri on the American frontier; it was located in present-day Sibley, Missouri. The Treaty of Fort Clark, signed with certain members of the Osage Nation in 1808, called for the United States to establish Fort Osage as a trading post and to protect the Osage from tribal enemies.