In April 1861, Wallace joined the Union army as colonel of the 11th Indiana Infantry. He initially served with his unit in the department of western Virginia. After he advanced to the rank of brigadier general in September 1861, he was involved in the capture of Fort Donelson. In March 1862, he was promoted to major general in March 1862. At the battle of Shiloh, he failed in his service to Gen. U.S. Gran. However, a year later, in the defense of Washington, he checked the Confederate advance at the battle of Monocacy. In 1865, he served on the committee that tried those responsible for the assassination of President Abraham Lincoln.
