In 1863 Forsyth transferred back to the Western Theater to serve as adjutant to General Philip H. Sheridan at Chickamauga. Forsyth was brevetted to Major in the Regular Army for his service Chickamauga. When Sheridan transferred to the Army of the Potomac's Cavalry Corps, Forsyth followed as his chief of staff. He participated in this capacity during the Overland Campaign. He was subsequently chief of staff of the Army of the Shenandoah during the Shenandoah Valley Campaign of 1864 and chief of staff to the Union Cavalries during the Appomattox Campaign. He received brevets to Lieutenant Colonel, USA for Cedar Creek, colonel, USA for Five Forks and Brigadier General, U.S. Volunteers, for the Shenandoah Valley Campaign.