As the Union Army under Major General William T. Sherman began to advance in Georgia in May 1864, Georgia Governor Joseph E. Brown appointed Carswell a brigadier general in the Georgia militia. Carswell led the 1st Brigade of the Georgia militia in opposition to the Union Army under Sherman during the Atlanta Campaign and Sherman's March to the Sea. The brigade fought at the Battle of Ruff's Station, now Smyrna, Georgia on July 4, 1864, the Battle of Atlanta on July 22, 1864 and during the subsequent siege of Atlanta until the fall of the city on September 2, 1864.