After fighting the admission of Kansas to the Union as a slave state, Ewing was elected the chief justice of the state supreme court in 1861. In the following year, he resigned from this position to join the army as a colonel of the 11th Kansas cavalry. His unit participated in the battles at Cane Hill and Prairie Grove in Arkansas before he was promoted to brigadier general in March 1863. He was assigned to the District of the Border, including Kansas. In 1864 when Confederate general Sterling Price led a raid into Missouri, he distinguished himself at the Battle of Pilot Knob and in the subsequent retreat. He resigned in February 1865. He was brevetted as a major general after the end of the war.
