The Lightning Brigade, also known as Wilder's Brigade or the Hatchet Brigade was a mounted infantry brigade from the American Civil War in the Union Army of the Cumberland from March 8, 1863, through November 1863. A novel unit for the U.S. Army, its regiments were nominally the 1st Brigade of Maj. Gen. Joseph J. Reynolds' 4th Division of Thomas' XIV Corps. Operationally, they were detached from the division and served as a mobile mounted infantry to support any of the army's corps. Colonel John T. Wilder was its commander. As initially organized, the brigade had the following regiments:92nd Illinois Mounted Infantry: Col. Smith D. Atkins 98th Illinois Mounted Infantry: Col. John J. Funkhouser (w), Lt. Col. Edward Kitchell 123rd Illinois Mounted Infantry: Col. James Monroe 17th Indiana Mounted Infantry: Maj. William T. Jones 72nd Indiana Mounted Infantry: Col. Abram O. Miller 18th Independent Battery Indiana Light Artillery: Capt. Eli Lilly

Mounted infantry were infantry who rode horses instead of marching. The original dragoons were essentially mounted infantry. According to the 1911 Encyclopædia Britannica, "Mounted rifles are half cavalry, mounted infantry merely specially mobile infantry." Today, with motor vehicles having replaced horses for military transport, the motorized infantry are in some respects successors to mounted infantry.