During the Mexican–American War, Wood served on the staff of General Zachary Taylor, and later joined the 2nd Dragoons. During the Civil War, he participated in many battles in the Western Theater. Wood commanded a division at the Battle of Chickamauga in September 1863, during which time he chose to obey a questionable order from commanding general William S. Rosecrans to reposition his division. In doing so, Wood opened a gap in the Union lines, which contributed to the vanquishing of the Federal forces in the battle and their subsequent retreat. Wood and his division took part in the successful Union assault during the Battle of Missionary Ridge in November during the Chattanooga Campaign. He also commanded the IV Corps of the Army of the Cumberland during the Battle of Nashville in December 1864.