In April 1861, Steedman became a colonel of the 14th Ohio Infantry. With this unit, he was involved in the battle of Philippi, in West Virginia. Steedman was sent to the western theater with a promotion to brigadier general. He fought at the battles of Perryville and Murfreesboro. In the Tullahoma campaign, Steedman distinguished himself as a division commander. At Chickamauga, he was recognized for heroism, but no promotion was forthcoming given his political loyalties. He was finally promoted in April 1864 to the rank of major general. In the Atlanta campaign, Steedman commanded a detachment at the Battle of Nashville. He resigned from the army in 1866.