This is the "body of raiding Federal horse" - i.e. a Union cavalry unit - that arrives at the McCaslin plantation sometime in 1862, causing the flight of Percival Brownlee (278).
Ike's account of the Civil War juxtaposes the leaders of the Confederate cause with the various Yankees who opposed them. This entry represents his roster of the economic elite: "the wildcat manipulators" and land speculators, "the bankers," the landlords and factory owners (273).
John Hunt Morgan commanded a cavalry regiment in the western theater of the Civil War, and, like Nathan Bedford Forrest, was known for his raids behind Yankee lines. His 1863 raid into southern Indiana and Ohio was the furthest Confederate penetration of the North. The novel's reference to "Morgan leading a cavalry charge against a stranded man-of-war" presumably refers to his burning of the Ohio River steamboat Alice Dean in Brandenburg, Kentucky (274).
Longstreet was one of Lee's corps commanders in the Army of Northern Virginia. Against his will, at the battle of Gettysburg he had to give the command to launch Pickett's charge against the Union army position on Cemetery Ridge. He was "shot out of saddle by his own men in the dark by mistake just as Jackson was" (273) - it happened at the battle of the Wilderness, and unlike Jackson, Longstreet survived.
