When the
American Civil War began in 1861, Bradley chose to enlist in the
Union cause. At first his mother opposed this, but he wrote her a letter in which he told of his need to serve in the military for his country. Under Maj. Gen.
Don Carlos Buell, he fought with his regiment at the capture of
Island Number Ten in
New Madrid, Missouri, and at the occupation of
Nashville, Tennessee. On October 15, 1862, he was promoted to colonel, and then he participated in the
Battle of Stones River that winter. Bradley was then appointed a brigadier general in the Union Army, to rank from July 30, 1864. He participated in the
Atlanta campaign, during which he commanded a brigade in Newton's Division of IV Corps in the
Army of the Cumberland. Bradley fought in
Tennessee at the
Battle of Spring Hill on November 28, 1864, and was once again wounded. His injury prevented him from leading his brigade during the
Battle of Nashville that followed on December 15–16. ==Postbellum==