U-84 was a Type VIIB, the workhorse class of the German submarine fleet. Launched on February 26, 1941, at the Flenderwerke shipyard in Lubeck and commissioned on April 29, she displaced 753 tons on the surface and 857 tons submerged. At 66.5 meters long with a beam of 6.2 meters, she was compact by warship standards but designed for endurance. Surfaced and running on her twin Germaniawerft diesel engines, U-84 could travel 8,700 nautical miles at 10 knots, enough range to cross the Atlantic and hunt for weeks before refueling. Submerged on electric motors, her range shrank to just 90 nautical miles at 4 knots, a limitation that forced U-boats to run on the surface whenever possible and made them vulnerable to aircraft. She carried 14 torpedoes fed to five tubes, four in the bow and one astern, plus an 8.8-centimeter deck gun and a 2-centimeter anti-aircraft gun. Her crew numbered between 44 and 60 men, depending on the patrol.

U-84's service record reads like a catalog of the Battle of the Atlantic's middle years. She joined her first wolfpack, Gronland, in August 1941, and over the next two years rotated through 16 more, each bearing the dramatic names the German navy favored: Markgraf, Schlagetot ("strike dead"), Raubritter ("robber knight"), Seewolf, Raubgraf. The wolfpack tactic was simple in concept and brutal in execution. Groups of submarines would spread across likely convoy routes, and when one boat made contact, it would shadow the convoy and radio its position to the others. The pack would then converge for coordinated night attacks on the surface, where the low-slung U-boats were nearly invisible. Across eight patrols, U-84 sank six Allied ships and damaged a seventh. The numbers sound modest, but each sinking meant cargo lost, crews killed or stranded, and war material that would never reach Britain or the Soviet Union.

In the summer of 1942, U-84 ventured far from the gray North Atlantic into the Gulf of Mexico. Under the command of Horst Uphoff, she joined Operation Drumbeat, the German campaign to attack shipping along the American coast, where defenses were still dangerously thin. On July 19, 1942, at 6:45 in the morning, Uphoff put a torpedo into the freighter Baja California just forward of midships. The ship was en route from New Orleans to Key West carrying general cargo that included glassware. She sank in about 114 feet of water, roughly 60 miles southwest of Fort Myers, Florida. The Gulf of Mexico campaign was devastating: during the first half of 1942, U-boats sank hundreds of ships in American waters, often within sight of shore, while oil slicks and wreckage washed up on tourist beaches. It was one of the worst Allied naval defeats of the war, and boats like U-84 were the instruments of that devastation.

By 1943, the Battle of the Atlantic had shifted decisively against the U-boats. Allied codebreakers were reading German naval communications. Long-range patrol aircraft closed the mid-Atlantic gap where submarines had once hunted without fear of air attack. Escort carriers joined convoy screens. And new weapons appeared, including the Mark 24 homing torpedo, which the Navy classified as a mine to prevent the Germans from learning that acoustic-tracking torpedoes existed. U-84's final patrol began in the spring of 1943, a period U-boat crews would later call "Black May" for the staggering losses they suffered. In May alone, 43 U-boats were destroyed. Uphoff and his crew survived into August, but on the 7th, a B-24 Liberator from American patrol squadron VB-105 caught U-84 on the surface in the North Atlantic. The aircraft dropped a Mark 24, and the weapon guided itself to its target. All 46 men aboard died.

The crew of U-84 left behind no famous last transmission, no dramatic rescue, no survivors to tell the story. They were 46 men, most of them young, who climbed into a steel cylinder and did not come back. The Battle of the Atlantic consumed roughly 30,000 German submariners out of the 40,000 who served, a casualty rate of 75 percent that made U-boat duty the most lethal assignment in any branch of any military in the Second World War. U-84's two years of service, from commissioning to destruction, spanned the arc of the entire submarine campaign: early success against poorly defended convoys, the Gulf of Mexico hunting season, the grinding attrition of the wolfpack battles, and the final technological reckoning when Allied detection and weapons outpaced every countermeasure the submarines could deploy. She rests now in the deep Atlantic, one of more than 700 U-boats lost during the war, each one a tomb.