Four months after the Pearl Harbor attacks, Kapitanleutnant Reinhard Hardegen decided that Americans should see for themselves what war with Adolf Hitler's Germany was going to look like. He began with Florida sunbathers. On April 11, 1942, Hardegen's submarine, U-123, torpedoed the tanker SS Gulfamerica off Jacksonville. He maneuvered U-123 around the flaming wreck and surfaced between the SS Gulfamerica and the beach. He sank it with U-123's deck gun. Hardegen later wrote in his log: "All the vacationers had seen an impressive special performance at [President Franklin D.] Roosevelt's expense. A burning tanker, artillery fire, the silhouette of a...