After the start of the Civil War, Mitchell served as the
Adjutant General of Kansas from May 2, 1861, to June 20, 1861. He later led the
2nd Kansas Infantry. Mitchell was badly wounded at the
Battle of Wilson's Creek on August 10, 1861, when his horse was shot while leading his regiment. While commander of the military District of Nebraska and in response to the Indian raid on Julesberg, Mitchell requested every ranch and military set the prairie ablaze to drive the Indians out. According to author and witness Eugene Ware, "They rolled as a vast confluent sheet of flame to the south.” The fire burned for more than three days along the banks of the Arkansas River in Kansas and Colorado with the flames reaching as far south as the Texas panhandle, with much of Nebraska south of the
Platte River and west of
Fort Kearny reduced to a blackened and desolate waste. ==Governor of New Mexico Territory==