At the Pentagon on the morning of 13 October 1950, Chief of Naval Operations Admiral Forrest Sherman was handed a message from the North Pacific. It began, “The U.S. Navy has lost control of the seas in Korean waters to a nation without a Navy, using pre-World War I weapons, laid by vessels that were utilized at the time of the birth of Christ.” The message originated from Rear Admiral Allen “Hoke” Smith, the longest serving line officer in the Korean War. As Commander, Task Force 95, Smith had been assigned to prepare the way for an amphibious landing of the First Marine Division at the port of Wonsan. He had just lost two minesweepers well east of the harbor channel and aerial reconnaissance indicated more mines lay ahead of his small minesweeping task group (TG). The mine threat at Wonsan threatened to delay D-Day for the Marines’ landing, just three days away.
The landing at Wonsan was intended to maintain the initiative retaken by United Nations forces against North Korea after the amphibious landing at Inchon the previous month. Securing the sizable port of Wonsan on the east coast of the peninsula would cut critical North Korean supply lines and open a second front against forces still reeling from the Inchon invasion. Admiral Turner Joy, then-Commander, Naval Forces Far East (ComNavFE), issued an operational plan emphasizing that a timely landing promised a double envelopment of North Korean forces, ensuring they could not return to the Republic of Korea (ROK) and offering the promise of their outright “military defeat.” The plan, however, identified that the “important problem to be solved” in preparing the landing would be clearing mines from the approaches to Wonsan. Admiral Joy gave Smith five days for the task while identifying the potential for a complex minefield.
Clearing the way for the landing would, instead, become an arduous 15-day operation. Minesweeping TG 95.6, under Captain Richard Spofford, would lose three vessels and suffer 200 casualties before the First Marine Division could go ashore. Spofford’s forces would overcome their limited size and resources and improvise their way into the port and beachhead at Wonsan, developing numerous mine countermeasure tactics still in use today. Through their valor and ingenuity, TG 95.6 would demonstrate that effective mine clearance operations require advance reconnaissance and technical intelligence, diverse and distributed clearance teams, and the ability to adapt rapidly under duress.
As the Navy and Marine Corps renew examination of littoral operations in contested environments, Wonsan is a reminder that even crude weapons can create decisive risk to operations when they deceive, surprise, or are employed en masse. Today’s naval mines, however, are no longer so crude. Accelerating advances in weapons autonomy, undersea communication, and sensor density herald a more complex future for the naval mine. Amid these developments, it is time to revisit the experience of TG 95.6 at Wonsan, where the U.S. Navy was held at bay by an enemy without a single capital ship.
Clearing the Way
As Commander, Mine Squadron Three, Captain Spofford had well-trained crews with experience in sweeping mines left by the Allies around Japan the previous year. The squadron, however, only had seven minesweepers of both steel and wooden hulls at the time of the landing. Spofford could count on Japanese and ROK support for 11 more. In contrast, the Okinawa landing had employed more than 100 minesweepers. By 1950, the Pacific Fleet no longer had a mine warfare-type commander, and limited technical and intelligence expertise on its staffs. Spofford himself had a great deal of experience in minelaying, but none in minesweeping before the trial at Wonsan.
When Mine Squadron Three got underway for Wonsan from Sasebo, Japan, on 6 October, intelligence on the North Korean mine threat at Wonsan remained limited. Mines recently had been sighted beyond the 100-fathom line there and North Korea was assessed to have mined Chinnampo to the north. Mines already had struck three U.S. vessels around the peninsula in the last month, including a strike that sank the USS Magpie (AMS-25). The bathymetry of the harbor was known, as was the location of a Soviet navigation channel. There was no specific intelligence, however, on the location, type, or number of mines that may lay in wait for TG 95.6. Spofford assessed that the Soviet channel marked on his charts was less likely to contain mines than the remainder of the harbor because of the recent retreat from the area. He did not believe, however, that he could meet the appointed D-Day by sweeping the length of the Soviet channel. Instead, he chose to clear the most direct route into the harbor from the 100-fathom line.
Sweeping commenced in the grey dawn of 10 October. The USS Pledge (AM-277) led a train of three minesweepers—the USS Incredible (AM-249), Mockingbird (AMS-27), and Osprey (AM-56)—to mark a channel 3,000 yards wide. Two more minesweepers trailed further behind, the USS Chatterer (AMS-40) to mark the channel with buoys and the USS Partridge (AM-16) to destroy any mines brought to the surface with gunfire. As sunlight faded over the harbor on 10 October, the minesweepers had cleared to the 30-fathom line but knew clearing further would be difficult. In the first operation of its kind, a helicopter from the USS Worcester (CL-144) conducted aerial reconnaissance and spotted five mine lines further in on the intended route.
At daybreak on 11 October, TG 95.6 began a second clearance effort focused on the port of Wonsan miles ahead of the minesweepers in the sea channel. Frogmen from underwater demolition teams (UDT) One and Three launched small boats from the USS Diachenko (APD-123) and USS Wantuck (APD-125) to reconnoiter the harbor. Their shallow-drafted landing craft personnel ramp (LCPR) boats traversed the harbor at high speed, receiving sporadic fire from North Korean forces ashore as they dropped and recovered swimmers from the frigid water. Most frogmen chose to abandon their newly issued wetsuits that proved too buoyant for them to readily dive beneath the surface and away from enemy fire. They swam in long johns, locating and marking mines hidden three to four feet beneath the surface with buoys. An early snow fell over Wonsan as the frogmen marked mine after mine, cut their mooring cables, and towed them out to sea for disposal. The frogmen also reconnoitered two outlying islands, Ung-do and Yo-do, in search of cables that would run to remote-controlled mines, but found none.
Back out at sea, Spofford chose to switch clearance routes and sweep through the Soviet channel. Fleet Air Wing Six had come through with PBM mariner patrol planes to assist. Their aerial reconnaissance seemed to foreclose the option of sweeping the most direct route into the harbor. There were simply too many mines. Ashore on the 11th, the 1st ROK Corps seized the port of Wonsan well ahead of schedule. The seizure reduced North Korean opposition to TG 95.6’s clearance efforts and relieved pressure on the urgency of the landing. The ROK’s victory also provided an opportunity to better understand the mine threat that lay between TG 95.6 and the beach. Spofford ordered Lieutenant Commander Don DeForest, a mine disposal officer just flown in from Mine Forces Atlantic, to go ashore to glean as much intelligence on the minefield as the ROK Army may could help him locate. At his midnight conference, with the ROKs in possession of Wonsan and DeForest planning to go ashore, Spofford was confident he could meet the appointed D-Day.
As day broke on 12 October, Spofford attempted another mine countermeasure innovation—a countermining air strike. F4U Corsair fighters from the USS Philippine Sea (CV-47) and the USS Leyte Gulf (CV-32) dropped 1,000-pound general-purpose bombs 200 yards apart for five miles in the Soviet channel. Spofford, along with experts from Mine Countermeasures Center Panama City, had calculated that a detonation within 30 feet of a mine would be sufficient for a sympathetic detonation. Limits to the precision of the airdrops left significant gaps in the line, however. Spofford believed the strike was ineffective, but that the concept of countermining air strikes was worthy of further study.
The minesweeping caravan added the USS Pirate (AMS-275) to the van and resumed work after the strike, advancing in the channel between the islands of Yo-do and Ung-do. They were soon back in mined water. Minutes later, the Pirate’s sweep gear brought six mines to the surface in quick succession. The Pledge then swept three with her port gear and the Incredible raised four more. As the mines on the surface multiplied, Lieutenant Commander Bruce Hyatt of the Pirate received word from a helicopter overhead that more mines lay dead ahead—followed by three successive mine lines.
Hyatt chose to turn back to the south and return to the Soviet channel, despite the risk of maneuvering in mined water. Mere moments into the turn, the Pirate was struck off the stern, lifting her out of the water and breaking her main deck. Within four minutes of the detonation, the ship slipped beneath the surface. Hyatt estimated that he was knocked unconscious for about 30 seconds and came to his senses again as the last of the water column from the explosion was striking the sea. He would report a quiet and orderly abandoning of the ship and that he and the ship’s executive officer stepped off just as the mast plunged beneath the waves.
Astern of the Pirate, the Pledge quickly cut her sweep gear to come to the Pirate’s aid. Suddenly, enemy shore batteries from the island of Sin-do opened fire. The Pledge’s crew returned fire as the enemy’s artillery found its range. Lieutenant Robert Young ordered a left full rudder to return to recover the Pirate’s survivors from swept waters. As she turned, a mine struck the Pledge amidships. Like the Pirate, the Pledge sank quickly as boats and frogmen from the Diachenko worked to rescue the surviving crew under withering fire from Sin-do.
The densest mine lines still lay ahead and clearance efforts had now been stymied on two routes into the harbor. A timely landing at Wonsan and the envisioned double envelopment stood at risk. It was then that Admiral Hoke Smith, with approval from Admiral Joy, fired off his alarming message to the Chief of Naval Operations. Sherman was sympathetic to the alarming assessment and made urgent plans to address mine warfare readiness from Washington.
Task Group 95.6’s clearance continued on 13 October, but without its steel-hulled minesweepers. The Incredible, the sole remaining steel-hulled ship, had lost both engines while recovering the crews of the Pirate and Pledge. Only four wooden-hulled “splinter fleet” minesweepers under Lieutenant Commander D’Arcy Shouldice, Commander, Mine Division-31, remained along with ROK and Japanese vessels. Light on minesweepers, Spofford augmented his force with all the small boats and frogmen he could assemble. Accompanied by a few sympathetic Koreans, the small boat crews took to marking mines spotted by helicopter and PBM crews under DeForest’s direction.
The small boat crews proved an efficient complement to the splinter fleet. By 15 October, Spofford was confident the sea channel was clear. He ordered the Diachenko to advance cautiously, using her sonar to identify any residual mine contacts. The Diachenko made it safely to the outskirts of Wonsan harbor just as Shouldice’s splinter fleet was completing final sweeps off the landing beaches. Then “the whole ocean started to erupt amidst the sweepers,” in the words of Lieutenant T.R. Howard of the USS Redhead (AMS-34). Three detonations came in quick succession—the third under the keel of the ROK YMS-516, shattering the ship. Shouldice reported they were not sure what type of mine had been triggered or how many lay in the harbor. Having been just hours from declaring the beachhead open, Shouldice now admitted, “We were back where we started.” His ships moved three miles off the beach and prepared for a slow and methodical sweep into shore to face a new challenge.
The same day, DeForest left the UDTs and finally went ashore to gather intelligence on the new threat. Working with the ROK 1st Corps and cooperative Koreans, he was able to draw a schematic of the remaining mine lines in the harbor. Eluding sniper fire and a near capture, DeForest was led to a haystack where he retrieved a metal coil from the sensor package of a mine that explained the detonations near the beachhead. There were magnetic influence mines sown along the beaches, but TG 95.6 now knew where they were—and how they functioned.
By 19 October, the splinter fleet was receiving assistance from the small-boat teams employing improvised sweep gear configured to detonate the magnetic mines DeForest had identified. The remaining frogmen continued to work to clear the final stretches of the harbor. Petty Officer Chet Bright, then a new member of UDT One, would later recall how his team would declare the harbor clear, only to return the following day and discover that new mines had been laid. The UDTs assessed that local fishermen were still laying mines at night despite ROK control of the port. They split duty between mine clearance in the daytime and all-night patrols to prevent the remining of the harbor, and would sink several enemy fishing vessels engaged in surreptitious mining before the harbor opened.
By the evening of 25 October, ten days after D-Day, Spofford could report that the channel, harbor, and beachhead were clear. TG 95.6 had removed 225 mines along a 25-mile route from a field of more than 3,000 mines spread over 400 square miles. Another 200 likely had broken their moorings and drifted with the currents. When the First Marine Division finally went ashore on 26 October 1950, they were greeted by Major General Almond of the U.S. Army’s Tenth Corps and by Bob Hope, who had arrived with a USO troupe for a show. The 50,000 Marines who arrived in the ensuing days passed a newly erected sign as they waded ashore: “This Beach is All Yours Courtesy of Mine Squadron Three.” The 50-year old mines had been beaten, but at significant cost, including keeping the First Marine Division from its planned double envelopment of the enemy.
Adapting Mine Countermeasures
After Wonsan, mine clearance operations would also be required at Chinnampo, Hung-nam, Kunsan, and Kojo. Minesweeping would continue until months after the armistice, including the resweeping of Wonsan, where residual enemy sympathizers proved adept at furtively mining a harbor under U.N. control. Mining would remain the chief threat to ships at sea throughout the conflict, causing over 70 percent of Navy casualties in the first two years of the war. The mine countermeasures force alone would account for 20 percent of all Navy casualties, despite constituting just two percent of the fleet.
The tactical adaptation critical to clearing Wonsan would be applied at Chinnampo the following month and captured in a Pacific Fleet assessment early in 1951. The report noted the efficacy of using UDTs and small, shallow draft boats for clearance operations. It recommended continued use of helicopters and PBMs for mine reconnaissance, to “look before you sweep” and leveraging amphibious vessels as “mother ships” to sustain helicopter and small boat operations. Further, the report aimed to correct deficiencies in early warning of mining, mine technical intelligence, and the integration of mine warfare expertise on fleet staffs.
Pacific Fleet lauded North Korea for keeping the U.S. Navy largely outside the 100-fathom line “with deterrent effects to naval gunfire support and delayed . . . logistic support” using only mines. Its report even suggested that future amphibious operations may be limited to only the most forbidding shorelines, where mining was least likely, surprise was possible, and ship-to-shore movement may be dependent on new helicopter capabilities. The report was a grave assessment of the potential for even antiquated and hasty mining operations to limit the Navy’s command of the sea.
As a result of Wonsan, Admiral Sherman brought renewed attention to mine warfare in the Navy. Concerned about the potential scope of the Soviet mining threat to command of the sea, commerce, and even U.S. ports, the U.S. Navy made significant peacetime outlays in mine warfare readiness and research. Sherman pointed to Smith’s infamous message in justifying the investment. “Hoke’s right: when you can’t go where you want to, when you want to, you haven’t got command of the sea. And command of the sea is the rock-bottom foundation of all our war plans. We’ve been plenty submarine-conscious and air-conscious. Now we’re going to start getting mine-conscious—beginning last week.” The buildup lasted only until 1958, and U.S. Navy efforts to sustain mine warfare readiness and competitive advantage have remained episodic.
Today’s mine countermeasures forces are far more advanced than those at Wonsan. Modern sonar, remotely operated vehicles, and unmanned systems technology have significantly increased the U.S. Navy’s capabilities in mine hunting and neutralization. Despite these advances, mine countermeasures remains a painstaking and risky task for which there is no technological panacea. As with any ambush weapon, naval mines have tremendous advantage once they lay in wait. These advantages are compounded when mines leverage the same new technologies as mine countermeasures forces. There is an almost inherent advantage to the offense in mine warfare. Mine-clearing forces can defeat these ambushes, but their deliberate pace of operations provide return on an adversary’s mining investment.
Taking the initiative against enemy mining, as suggested in the Pacific Fleet assessment, requires acting before mines enter the water. A fleet that operates with speed, deception, standoff, dispersal, and early warning of mining can mitigate the risks of another Wonsan. Offensive operations against minelaying and superb maritime domain awareness can further mitigate the naval mine’s advantages. Once laid, mine technical intelligence is critical to providing clearance teams an advantage—just as with DeForest’s magnetic coil.
Modern weaponry’s increasing autonomy and sensor density portend a more lethal future for the naval mine in the littorals and in blue water. To meet these challenges, future fleet design and operational concepts must consider the defensive and offensive demands of mine warfare. The ability to fight and win in a mined environment demands reflection on the structure of Wonsan’s clearance teams and their capacity for rapid adaptation if the U.S. Navy is to mitigate the enduring advantages of weapons that wait for prey from beneath the waves.
 Tamara Moser Melia, “Damn the Torpedoes: A Short History of Mine Countermeasures, 1777 – 1991.” Naval Historical Center, Washington, D.C., 1991, p. 76.
 Commander-in-Chief, U.S. Pacific Fleet, “Interim Evaluation Report No. 1, 25 June to 15 November 1950,” Volume 5. Naval Historical Center, Washington, D.C., p. 832. Hereafter “CINCPACFLT IER.”
 Joseph H. Alexander, Fleet Operations in a Mobile War: September 1950 – June 1951,” Naval Historical Center, Washington, D.C., 2001, p. 28.
 Malcolm W. Cagle and Frank A. Manson, “Wonsan: Battle of the Mines,” Proceedings, June 1957, https://www.usni.org/magazines/proceedings/1957-06/wonsan-battle-mines.
 Malcolm W. Cagle and Frank A. Manson, The Sea War in Korea, Annapolis: Naval Institute Press, 1957, p. 134 – 135.
 Alexander, p. 29.
 Philip E. Carrico, “Naval Frogmen in Korea: Play Taps for Pledge and the Pirate,” Military: World War II, Korea, Viet-Nam & Today, January 2006, p. 22 – 23.
 Chet Bright with Derek Turner, Bluejacket: A Sailor’s Life, Kentucky: Bowling Green Publishing, 2012, p. 53
 The Sea War in Korea, p. 137.
 The Sea War in Korea, p. 136.
 Malcolm W. Cagle and Frank A. Manson, “Errors of the Korean War,” Proceedings, March 1958, https://www.usni.org/magazines/proceedings/1958-03/errors-korean-war. Debate would follow the landing at Wonsan as to whether the risk assumed was worth the cost of the clearance operation. Admiral Joy and Admiral Arthur “Dewey” Struble, Commander, Joint Task Force SEVEN, had both opposed the concept of the landing even before its approval. Joy was concerned about dividing Marine forces when the offloading at Inchon was still underway. Struble, after extensive minesweeping experience in World War II, was concerned that the delay from mine clearance would make the landing irrelevant. MacArthur would later say their opposition never reached his ear.
 The Sea War in Korea, p. 136.
 The Sea War in Korea, p. 137.
 Battle of the Mines.
 “U.S.S. Pirate (AM-275); Report of Loss.” C. E. McMullen and B. Hyatt to Secretary of the Navy. October 19, 1950. Naval Historical Center, Washington, D.C. LT McMullen was the Commanding Officer of the Pirate; Hyatt was aboard as the Commander, Mine Division-32.
 The Sea War in Korea, p. 139.
 Bright, p. 54.
 George F. Eliot, “Russia Threatens our Seapower,” Collier’s Magazine, September 1953, p. 33.
 The Sea War in Korea, p. 143.
 Melia, p. 77. DeForest would receive a Silver Star for his command of the small boats during the clearance operation and his intelligence operation ashore.
 Chet Bright with Derek Turner, Bluejacket: A Sailor’s Life, Kentucky: Bowling Green Publishing, 2012, p. 53.
 The Sea War in Korea, p. 144.
 Melia, p. 82.
 Eliot, p. 36.
 Melia, p. 79 – 80.
 CINCPACFLT IER, Vol. II, p. 103 – 109.
 CINCPACFLT IER, Vol. II, p. 102.
 CINCPACFLT IER, Vol. V, p. 714.
 Eliot, p. 32.
 Melia, p. 133.
 Gregory Hartmann, Weapons that Wait: Mine Warfare in the U.S. Navy, Annapolis: Naval Institute Press, 1979, p. 229 – 230.
 Timothy McGeehan and Doug Wahl, “Flash Mob in the Shipping Lane,” Proceedings, January 2016, p. 52 – 53.