Outside, a heavy summer rainstorm thrashed the streets of Richmond, Va. Inside Libby Prison that afternoon — July 8, 1863 — hundreds of captured Union officers sprawled across the bare wooden floors of the converted tobacco warehouse, picking lice out of their underwear and passing around copies of Richmond newspapers. The news was grim for the Yankees: Gen. Robert E. Lee’s Confederate army had defeated Gen. George Meade’s Union troops at a Pennsylvania town called Gettysburg. “OUR ARMY AGAIN VICTORIOUS,” read the headline in The Richmond Examiner, “MEADE’S ARMY ANNIHILATED.” “THE ENEMY ROUTED,” reported the Richmond Dispatch. “FORTY THOUSAND PRISONERS...