WASHINGTON—When Herbert G. Claudius's family would ask him if he'd ever sunk an enemy submarine during his decades in the U.S. Navy, Claudius would say that he thought he did once. He'd seen oil and debris after a fierce battle he'd led against a German U-boat in the Gulf of Mexico in 1942.

But Claudius could never be sure that he'd sunk the sub.

The U.S. Navy certainly didn't seem to think so. After the battle, just 25 miles (40 kilometers) from the U.S. mainland, the Navy removed Claudius from command and sent him to anti-sub-warfare school.

But on Tuesday, Claudius was posthumously vindicated at the Pentagon, thanks in part to exploration supported by the National Geographic Society. The U.S. Secretary of the Navy announced that his ship had indeed fired the depth charges that sank German U-boat U-166.

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"Seventy years later, we now know that [Claudius's] report after the action was absolutely correct," Navy Secretary Ray Mabus said in a small ceremony attended by members of Claudius's family.

"[Claudius's ship] did sink that U-boat, and it's never too late to set the record straight," Mabus said, as he presented the late captain with a posthumous Legion of Merit with a Combat "V" device, which recognizes heroism in battle.

Claudius's son, Gordon Claudius, accepted the medal and said that he wished his father could have known about the correction to a largely forgotten chapter in American history. Claudius passed away in 1981, after a 33-year career in the Navy.

"He would have felt vindicated," Gordon Claudius said.

U-boat expert Richie Kohler, who also attended the ceremony, put it more bluntly. "Claudius was shafted," Kohler said. "He should have returned home a hero, but he was humiliated and sent back to school."

The new understanding of Claudius's role is based on recent exploration of the wreck of U-boat U-166 off the coast of Louisiana, supported by a grant from the National Geographic Society. The wreck was discovered by an oil company in 2001, under 5,000 feet (1,500 meters) of water.

Last summer, oceanographer and National Geographic Explorer-in-Residence Robert Ballard returned to the site with remotely operated vehicles to conduct high-resolution mapping and try to reconstruct what happened. (Learn about a deep wreck found off the U.S.)

View Images An undated file photo shows the Robert E. Lee, a passenger ship that was sunk by German submarine U-166 on July 30, 1942, in the Gulf of Mexico. The U-boat was subsequently destroyed by an American patrol boat. Photograph by HO C&C Technologies, Inc., Associated Press

Unbeknownst to Most Americans

In the summer of 1942, Claudius was a newly minted captain of a brand-new patrol ship, PC 566.

He was assigned to escort the passenger ship Robert E. Lee with PC 566 in the Gulf of Mexico. The Robert E. Lee was transporting hundreds of survivors of other U-boat attacks to New Orleans.

During the early years of World War II, the U.S. was ill-prepared to defend its coasts, and the technologically advanced U-boats exploited that weakness by disrupting shipping.

Unbeknownst to most Americans at the time, German U-boats had been sinking civilian and military ships just off the coast.

Parts of the U.S. coast were so defenseless that "at first it was like a turkey shoot," Ballard told National Geographic.

View Images A gun can still be seen on the deck of the U-166, well preserved in the deep water. Photograph by Ocean Exploration Trust

Over 20 U-boats sank more than 70 ships in the Gulf of Mexico between 1942 and 1943. The Germans called it "Operation Drumbeat" or the "Second Happy Time," following up on previous attacks on the United Kingdom.

The raids on the U.S. were rarely discussed by leaders or in the media, for fear of spreading panic.

On July 30, 1942, U-boat U-166 attacked the Robert E. Lee southeast of the mouth of the Mississippi. The passenger ship sank, taking 25 lives (404 survived). Claudius and his crew fought back with guns and depth charges.

Oil and debris rose to the surface and they didn't see the U-boat again, so captain and crew reported that they thought it had been destroyed.

The Navy had doubts about the crew's report, especially since Claudius and the men had not yet received anti-submarine training. Things got more muddled when, a few days later, a patrol plane shot at a U-boat nearby. The pilots thought they sunk it.

Record keepers assumed the aviators must have destroyed U-166, but communications later revealed that there were multiple U-boats in the area—and that the one engaged by the plane was only slightly damaged.

But it wasn't until this week that PC 566 was credited with making the U-166 kill.

View Images All crew of the U-166 were lost, and the wreck is now considered a war grave that cannot be disturbed. Photograph by Ocean Exploration Trust

Mapping the Wrecks

Ballard and Kohler used remotely operated vehicles to explore and map the wreck of U-166, which lies near the wreck of the Robert E. Lee, just as Claudius had predicted. The scene is surrounded by the remains of lifeboats and shards that broke off the submarine.

The U-boat's stern is intact and highly preserved thanks to the great depth at which it was resting, says Ballard, and still features a gun on its deck. The rusticles, or rust icicles, on the structure reminded him of his discovery of the Titanic. (See five shipwrecks explorers would love to find.)

"The conning tower looks like you could knock on the door," Ballard says, referring to the raised platform on the top of the sub.

View Images The conning tower of the submarine is still visible, though the bow of the vessel was shattered. Photograph by Ocean Exploration Trust

But the bow had been blasted into fragments, suggesting the work of a depth charge, he added. Ballard's exploration of the site will be featured in an upcoming NOVA/National Geographic documentary.

Kohler says the bow's damage shows that Claudius "did exactly what he said he did."

The wrecks are protected as war graves and will remain the final resting places for the German and American dead.

At the Tuesday ceremony at the Pentagon, Mabus praised Claudius and his crew for braving "very dangerous waters."

"It was a very tough time," Mabus said.

"Nazi Attack on America" is a NOVA/National Geographic special presentation, premiering Wednesday, May 6 at 9pm ET/8c on PBS.