Forney resigned his commission on January 23, 1861, and entered the Confederate Army as
colonel of the Alabama Artillery, then as a captain of infantry on March 14, 1861 and colonel of the
10th Alabama Infantry in the
Army of the Shenandoah on June 21, 1861. He took command of the 5th Brigade and other Confederate forces in the Shenandoah Valley on June 21, 1861 while the remainder of that army moved to participate that same day at the
First Battle of Bull Run until October 1861. He was wounded at the
Battle of Dranesville, Virginia, on December 20, 1861. Forney was promoted to
brigadier general on March 10, 1862, and to
major general on October 27. After service as commander of the Departments of Alabama and West Florida and the District of the Gulf Department No. 2 in 1862 and the 2d Military District Department of Mississippi and East Louisiana in early 1863, he was given command of a division of
Lieutenant General John C. Pemberton's
Army of Mississippi defending
Vicksburg in April 1863. He was captured there when the city fell in July 1863. After being exchanged on October 13, 1863, Forney was a division commander in the Department of Mississippi and East Louisiana until January 28, 1864, then in the Department of Alabama, Mississippi and East Louisiana until May 1864. After a period without a command, he followed
John George Walker as commander of the
Texas Division, Division 1 of I Corps, Trans-Mississippi Army in September 1864. From March 27, 1865 until May 12, 1865, he commanded a division in the Department of Texas, New Mexico and Arizona. ==Post-war and death==