Convinced that war was inevitable, the winter of 1860-61 found him studying military tactics and engineering.  When the call came for troops, he was the first man to enroll, and largely through his efforts Company H of the 20th Regiment, Illinois Infantry, was raised in Putnam County.  When the regiment was organised at Joliet, Illinois, he was appointed sergeant-major, and in this capacity went to the front.  When the force was sent to Cape Girardeau, Missouri, his prescience in studying military engineering made him invaluable.  He was practically given charge of planning and laying out and constructing the fortifications at that place, a work he executed so well that it received the unqualified commendation of General Fremont.  The second lieutenant of Company H resigning, Powell was elected to fill the vacancy.  After a service of a few weeks with his company he was put in charge of the fortifications he had constructed, being retained in this post after the departure of his regiment.  In the early winter of 1861-62 he recruited a company of artillery, largely from loyal Missourians.  This company was mustered into service as Battery F, 2d Illinois Artillery, John Wesley Powell, Captain.  After drilling a few weeks he was ordered to proceed with his battery to Pittsburg Landing, Tennessee, where he arrived the latter part of March, 1862.  The battery took part in the battle of Shiloh, April 6th of that year, and during the engagement, as Powell raised his arm, a signal to fire, a rifle ball struck his hand at the wrist glancing toward the elbow.  The necessary surgery was done so hastily that later a second operation was imperative, which left him with a mere stump below the elbow-joint.  Never for long at a time afterward was he free from pain and only a few years ago a third operation was performed which brought relief.

As soon as the original wound was healed he went back to his command, assisting as Division Chief of Artillery in the siege of Vicksburg.  After the fall of this place he took part in the Meridian Raid.  Then he served on detached operations at Vicksburg, Natchez, and New Orleans until the summer of 1864, when he was re-assigned to the former command in the Army of the Tennessee.  In all the operations after the fall of Atlanta he bore an active part, and when Sherman commenced the march to the sea, Powell was sent back to General Thomas at Nashville, in command of twenty batteries of artillery.  At the battle of Nashville he served on the staff of Thomas and continued with this command till mustered out in the early summer of 1865.  As a soldier his career was marked by a thorough study and mastery not only of the details of military life, but of military science.  Especially was he apt in utilising material at hand to accomplish his ends—­a trait that was also prominent in his civil life.  Bridges he built from cotton-gin houses, mantelets for his guns from gunny bags and old rope, and shields for his sharpshooters