Complex sentence: After leaving Oregon he went to Washington, D.C. and worked for the Bureau of Indian Affairs as an agent, with involvement in that capacity dealing with the Santee Sioux in the Iowa Territory during the Inkpaduta affair in 1857.
Split into simpler sentences: After leaving Oregon he went to Washington, D.C. and worked for the Bureau of Indian Affairs as an agent. In that capacity, he dealt with the Santee Sioux in the Iowa Territory during the Inkpaduta affair in 1857.