A year later, O'Fallon supplied Clark's military expedition to Prairie du Chien in present Wisconsin.

O’Fallon said, "It will be a vain struggle to attempt the change of the treacherous savage, so long as un-principled British Traders are permitted to trade within our Territory.

At St. Louis, he turned him over to his uncle William Clark (then the governor of Missouri Territory), who released Dickson and had him returned to Canada.

[11] Atkinson and O'Fallon had differences of opinion over Native American policy and the degree to which a military commander held dominion over the Indian agent.

It was initially a difficult job to catch and kill them, but a steel-cage trap was invented in 1823 that greatly simplified the process.

William Henry Ashley advertised for 100 men to join his firm to trap for beaver along the Missouri River.

O'Fallon saw this as an outrageous plan that would undercut white traders and disturb the peaceful and collaborative commerce with local tribes.

A fight broke out that led to a dawn attack on Ashley's group of 90 trappers by 600 natives called the Arikara War.

The 6th Infantry Regiment rode out on June 22 to avenge the attack, which O'Fallon believed was instigated by the British.

[14] President James Monroe signed a bill on May 25, 1824, that created the Bureau of Indian Affairs and provided the funds for a peace commission to travel up the Missouri River, at the cost of $10,000 (equivalent to $277,455 in 2023).

[1] Each treaty-signing ended with an awe-inspiring military parade, the firing of cannons, and the distribution of whiskey… The only rancorous notes came from Benjamin O’Fallon, who continued to inveigh against the 'Continued robbing and butchering of our people by the Indians' and the 'dark designing sycophants' who protect them.

George Kennerly stepped in to prevent an attack, and he later stated that he was embarrassed that O'Fallon was the "nephew of the man whom I look up to as a father.

[18] Benjamin O'Fallon moved to the Sulphur Springs area where he established the Indian Retreat Plantation in 1834.

[19] He built gristmills, operated a small plantation, and sought other sources of income, but he did not become wealthy like his brother John O'Fallon.

His house at Indian Retreat Plantation "was a veritable museum of Native American material culture".

George Catlin wanted to build a national Indian Gallery of each of the tribes, with people in traditional clothing.

[9][22] A patron of the arts,[9][21] O'Fallon commissioned or purchased portraits and landscapes from Catlin,[9][21][22] acquiring 42 paintings.

The works of art stayed within the O'Fallon family until 1894 when 35 paintings were purchased by the Field Museum of Natural History in Chicago.

[24] During the early 1830s, O'Fallon purchased a partial skeleton of a fossil marine reptile from a fur trapper.

He purchased the fossil and delivered it to University of Bonn naturalist Georg August Goldfuss for research, who published a study in 1845.

[28] His correspondence are among the O'Fallon Family Papers at the Beinecke Rare Book and Manuscript Library, Yale University.