The remnants of Fort Miles amid shifting dunes in Delaware’s Cape Henlopen State Park tell a tale of wartime vigilance and later, Cold War espionage. The fort, originally covering 1,550 acres, stands as a multifaceted embodiment of America’s mid-century coastal military architecture.

Guarding the Industrial East Coast

Named after U.S. Army General Nelson A. Miles, who served in the Civil War, the Spanish-American War, and the Indian Wars, Fort Miles was established as part of the 1940 Harbor Defense of the Delaware. Its primary role was to protect Delaware Bay, a vital maritime access point to the East Coast’s industrial centers. In 1941, the fort became the base for the 261st Coast Artillery, tasked with defending the E.I. duPont de Nemours & Co. complex in Wilmington, the city of Philadelphia, and the oil refineries along the Delaware River in Pennsylvania and New Jersey.

Fort Miles at its height was a small town of 2,500 men and women working to protect its Delaware Bay charges, one of the largest seacoast fortifications built in the United States.

The fort’s construction included the monumental Battery 519, a 420-foot underground bunker that housed formidable 12-inch coastal defense guns. Yet, this bunker was merely one limb of a larger octopus. The sprawling complex incorporated several batteries, bunkers, barracks and a network of fire control towers.

Nineteen towers, erected along both the Delaware (15) and New Jersey (4) coastlines, were the eyes of Fort Miles, designed to spot and triangulate the coordinates of enemy surface ships. If tower spotters saw a surface ship, they would call the battery plot room. Working with mechanical computing techniques, men in the plot room calculated wind speed, the Earth’s curvature and tower coordinates. Then soldiers in the plot room would tell the crew on the 12-inch gun the trajectory to use in firing a 1,100-pound shell that could fly 15 miles.

The Threat Below—Nazi U-Boats

However, as the United States found itself ensnared in World War II following Germany’s declaration of war in December 1941, the existential threat evolved. Fort Miles soon realized that the real menace lurked beneath the water’s surface: the U-boats of Nazi Germany’s Kriegsmarine.

Under the direction of German Admiral Karl Doenitz, who commanded a fleet of nearly 1,200 U-boats during the conflict, these “undersea boats” prowled the Atlantic seaboard, targeting Allied shipping lanes and coming alarmingly close to American shores. And they were a menace: In the war’s early days, German subs sank 25 ships totaling 200,000 tons. No U-boat was damaged.

In response, Fort Miles expanded its defensive operations to include anti-submarine measures. Thirty-six types of artillery were available to the base’s soldiers. Yet, despite vigilant watch, the GIs could do little against submerged threats. This underlined the complex theater of operations and Fort Miles’ role in it—a lookout post that could do little more than observe and sound the alarm.

In the last seven weeks of the war, Doenitz sent six submarines into the Atlantic to blitz Allied shipping. The Navy set up ‘Operation Teardrop’ — four aircraft carriers, 112 planes and 42 destroyer escorts — to seek out the pack. Operation Teardrop had a special urgency because United States intelligence agents had heard a rumor that the subs carried new, long-range “rocket bombs” that could easily strike East Coast industrial cities. (No such rockets were ever found.)

U-858 surrenders

Four U-boats were sunk in April, one surrendered in Maine and U-858 “eluded the dragnet.” In the turbulent days following V-E Day, on May 14, 1945, Lt. Cmdr. Thilo Bode and the crew of U-858 finally surfaced off Cape Henlopen to surrender. This marked the first foreign surrender on U.S. soil since the War of 1812.

U-858 was the 2nd to last U-Boat surrender at war’s end. The 57-man crew surrendered first to the Marines, who boarded her; they had to re-enact the surrender for the admiral, and they had to surrender to the Army because the Army didn’t recognize a surrender to the Navy.

Bode was interrogated in Washington and the full crew was released in 1948. The U.S. Navy kept the U-858 and charged Americans to tour it for war bond drives.

The Navy sunk U-858 in October 1947 off Cape Cod during torpedo tests. The only remaining U-boat in the United States, U-505, is a museum piece in Chicago. U-505 was captured at sea by the Navy in 1944.

Fort Miles adapted to the new geopolitical landscape in the post-war years. It morphed into a Navy underwater listening post as part of the Sound Surveillance System (SOSUS) aimed at detecting Soviet submarines. When SOSUS identified an underwater threat, Navy planes would drop sonar buoys, amplifying their ability to monitor Soviet activities.