Marmaduke was on duty in the
New Mexico Territory in the spring of 1861 when he received news that
several southern states had declared secession from the
United States (
Union). He returned home to Missouri to meet with his father, a strong
Unionist. Afterward, Marmaduke resigned from the United States Army, effective April 1861.
Pro-secession Missouri Governor
Claiborne Fox Jackson, Marmaduke's uncle, soon appointed him as the colonel of the 1st Regiment of Rifles, a unit from Saline County, Missouri, in the
Missouri State Guard. Marmaduke was wounded in action at the
Battle of Shiloh as colonel of the
3rd Confederate Regiment, incapacitating him for several months. In November 1862, the C.S. War Department confirmed Marmaduke's promotion to brigadier-general. His first battle as a
brigade commander was at the
Battle of Prairie Grove. In April 1863, he left Arkansas with 5,000 men and ten
artillery pieces and entered U.S.-held Missouri. However, he was repulsed at the
Battle of Cape Girardeau and forced to return to
Helena, Arkansas. Controversy soon followed. In September 1863, he accused his immediate superior officer, Brigadier-General Lucius M. Walker, of cowardice in action for not being present with his men on the battlefield. Walker, slighted by the insult, challenged Marmaduke to a duel, which resulted in Walker's death on September 6, 1863. Marmaduke later commanded a cavalry division in the Trans-Mississippi Department, participating in the
Red River Campaign. While commanding a mixed force of Confederate troops, including Native-American soldiers of the 1st, and 2nd Choctaw Regiments, he defeated a U.S. foraging detachment at the
Battle of Poison Spring, Arkansas, on April 18, 1864. After the battle, his troops
massacred the black soldiers of the
1st Kansas Colored but not the men of the white Union units. He was hailed in the Confederate press for what was publicized as a significant Confederate victory. Marmaduke never was prosecuted for this war crime. Marmaduke commanded a division in Major-General Sterling
Price's Raid in September–October 1864 into Missouri, where Marmaduke was captured at the
Battle of Mine Creek in Kansas (by Private
James Dunlavy of the 3rd Iowa Cavalry). While still a
prisoner of war at
Johnson's Island in
Ohio, Marmaduke was promoted to major-general in March 1865. He was released after the war ended. His younger brother, Henry Hungerford Marmaduke, who was in the
Confederate States Navy, was captured and imprisoned on
Johnson's Island. He later served the U.S. government in negotiations with South American nations. He was buried in
Arlington National Cemetery. Two other Marmaduke brothers died in the American Civil War. == Later life and death ==