1944 - First use of torpedo squadrons from carriers to drop aerial mines (Palau Harbor)There is a connection.
1972 - Easter Offensive began in Vietnam
A brief review of the history of sea mines
War supports innovation. Among such innovations was the sea mine. Diana Schroeder's "The History of the Sea Mine and its Continued Importance in Today's Navy" (which can be found at the Naval Undersea Museum's website here) notes various forms of sea mines in an easy to digest writing.
Early mines were designed such that they would explode on contact with the hull of a ship, or through a timing mechanism or through a "command" delivered from shore. Mines were traditionally placed by ships, submarines or from shore. Given their relatively low cost, they have proven to be remarkably effective weapons.
World War II saw many new innovations involving mines, not the least of which was the ability to sow sea mines from aircraft, as reflected in Mines Away! The Significance of U.S. Army Air Forces Minelaying in World War II by Major John S. Chilstrom (hereafter "Chilstrom" and available here (pdf- all footnotes from the original have been omitted herein)):
The first recorded aerial minelaylng in combat occurred on November 20. 1939, when nine Heinkel 59 floatplanes flew to the Thames Estuary. Although five turned back due to navigation difficulties, four aircraft laid seven mines that night and thirty-four more in the following two days.This aerial drop also lead to recovery of some unexploded mines with a new type of detonator, based on magnetism (see the images of the German sea mine which were found here) This, in turn, as noted here, led to use of degaussing to counteract magnetic mines.
Air Delivered Mines
Degaussing aside, the key point is that aerial mining opened up a huge number of new areas that could now be mined - areas previously unreachable by surface ships or submarines. These included areas like the Thames estuary and other inland rivers, canals and waterways. Chilstrom reports that the British embraced this new delivery method:
The British accomplished almost all allied mining in the European theater. The Royal Air Force (RAF) quickly adapted to the task, and Bomber Command laid 47,307 mines--eighty percent of the total offensive effort. In 1936, an officer in the Air Ministry had been the first to advance the idea of air--dropped magnetic mines. In 1939 that same airman commanded No.5 Group--the sole bomber unit charged with aerial minelaying. He was Arthur T. Harris, who quickly ensured all Bomber Command aircraft could carry mines once he became commander-in-chief in 1942. From the first few mines laid by his Handley Page Hampdens on April 13, 1940, Harris raised the number delivered to just over 1,000 in 1941, and then an average of 1,000 per month for the rest of the war."Bomber" Harris went on to fame for his leadership in strategic bombing, though his role in mine warfare seems to be less well remembered, at least by Americans.
Bomber Command dropped mines along the Norwegian coast, in the Baltic Sea, Heligoland Bight, the Bay of Biscay, and the Mediterranean along the Italian-Sicilian coasts. As an example of tactical support, the RAF laid nearly 4,000 mines between April and June 1944 to prevent interference with the Normandy landings. The British also mined inland waterways, in particular the Kiel and Koenigsberg Canals, and the Danube River. Thus, the objectives of minelaying varied, from threatening warships and U-boats, to disrupting Germanys ability to import raw materials, transport supplies, or move troops.
How successful were these efforts? Chilstrom:
For the effort, (about five percent of Bomber Command sorties), the RAF could eventually claim 762 Axis ships sunk and 196 damaged.(Altogether, British mines in the European theater totalled 260,000--mostly defensive--and ship losses from them numbered 1,590). In addition to sinking ships, the mines disrupted sea communication and blocked transportation on inland waterways. Another effect was tying up a large German minesweeping force, which comprised forty percent of all German Navy activity by 1945.The U.S. was slower to accept aerial mining - though the British were generous in sharing their technology. Chilstrom's Chapter 3 has a nice discussion of the politics of why the U.S. lagged in mine warfare which seems mostly to have been due to simple ignorance and prejudice. As aerial mining came into the Pacific Theater, about 60% of that mission was performed by the Royal Air Force (RAF) and Royal Australian Air Force (RAAF) with the remainder by the USAAF and, as noted at the beginning, in late March 1944, the Navy. When used, the results were excellent:
Southwest Pacific mining supported the strategic anti- shipping campaign by hindering the resupply of Japanese garrisons and the flow of resources to the home islands. In this theater the U.S. Seventh Fleet commander, Vice Admiral Thomas C. Kinkaid, directed the aerial mining almost exclusively conducted by the RAAF. His goal was to cause the most havoc possible given the limited number of mines and aircraft available. This caused him to disperse the effort, never saturating anyone port, but still hurting the Japanese in terms of ships lost and minesweeping required. Kinkaid had no doubts about the value of aerial mining. In a letter to the CNO in July 1944 he claimed, "Aerial mining operations were of the order of 100 times as destructive to the enemy as an equal number of bombing missions against land targets."But what about the Navy?
The PBY-5 Catalinas used by the RAAF were amphibious aircraft that provided good results. The aircraft was well suited to minelaying, with long range and a payload of 2,000-4,000 pounds. Out of 1,130 successful sorties that laid 2,498 mines, the Australians lost nine aircraft, a 0.8 percent loss rate. Altogether, the postwar U.S. Strategic Bombing Survey estimated these mines sank 90 ships total ling 250,000 tons, or approximately 40 percent of Japanese losses in the Netherlands East Indies.
On August 10, 1944, fourteen B-29s laid their first mines on a mission from China Bay, Ceylon to the refineries at Palembang, Sumatra. The mines sank or damaged seven ships and closed the Moesi River entrance to tankers for a month.
Carrier based TBF Avenger torpedo-bombers occasionally used mines to support anti-shipping air strikes and amphibious landings. When naval aircraft used mines, the objective was a direct, immediate and synergistic effect with another form of attack.In short, the mines were used to "cork" the bottleneck port entrance and set up the follow on "ducks on a pond" missions.
TBFs of Carrier Task Force 58 made the first American carrier-based minelaying at Palau on March 30-31,1944. They trapped thirty-two ships inside the atoll by mining its passages, allowing aircraft to sink twenty-three with bombs and torpedoes and damage the rest. That action, and additional mining, led the Japanese to abandon Palau as a base.
It seems, however, that aerial mining, though a Naval capability, was primarily an Air Force mission. The results showed that it was an effective tool in the commander's box:
Of the approximately 13,000 mines laid in Japans "outer zone," aircraft dropped 9,254 from 3,231 sorties to create 108 minefields. Across this large area, aerial mining sank or damaged as many as 405 ships amounting to 776,260 tons at a cost of 40 Allied aircraft.6 Though difficult to measure, the Bombing Survey stressed, "Even more important was the fact that vita] shipping was greatly hampered in its movements and delayed for periods ranging from a day or two to a month."7 After some ports were mined, such as Rangoon and Haiphong, large ships seldom visited them, while mines frequently closed numerous others, such as Shanghai, Hong Kong, Takao, Bangkok, Singapore, Balikpapan, and Surabaya. Mining a1so convinced the Japanese Navy to abandon anchorages at Pa1au, Penang, and Kavieng. The disruption of merchant traffic, in turn, caused supp1y problems for the Japanese Army deployed in China, Burma, Siam, Malaya and Indochina. The Bombing Survey specifically credited the mining of an in]and waterway, the Yangtze River, with having a blockading effect that significant]y hampered Japanese Army offensives in China during 1944-45.8 This action prompted the Survey to suggest: "The successes obtained helped prove the value of the aerial mine as an air force weapon."A mention of the psychological effect of sea mining by air during WWII can be found here.
Those who remembered its effectiveness called for the use of aerial mining in later wars:
Early in the Vietnam war, many American military leaders advocated mining since most of the Norths military and economic assistance came by sea. In fact, a full 85 percent of the country’s imports passed through the sing1e port of Haiphong. In a postwar interview, Admiral Thomas H. Moorer said. "I think both the Johnson and Nixon administrations were about eight years late. I first recommended mining Haiphong in 1964 when I was Commander-in-Chief of the Pacific Fleet." Moorer added, "In my opinion, the failure to mine Haiphong immediately was the difference between winning and losing in Vietnam." l8 Due to the character of the war in Southeast Asia at that time (substantially a guerrilla war fought in the South), the efficacy of mining so early in Americas inv0lvement is questionable. However, it was political restraint, motivated by the United States negative objective of preventing Soviet or Chinese intervention, that then prohibited a large-scale mining effort.Prohibited, that is, until 1972, when the other event mentioned above, the invasion of South Vietnam during the "Easter Offensive":
In the spring of 1972, the North Vietnamese mounted a conventional invasion of the South by twelve divisions which proved highly vulnerable to air power. Meanwhile, the United States relations with the Soviets and Chinese had so improved that President Richard Nixon could direct the mining of North Vietnams ports without fear of intervention.The mining effort is well-described here:
After years of awaiting the order, carrier attack aircraft would now close Haiphong with mines. While the planes were airborne, the president announced that ships in port had three days to leave before the mines activated. Five elected to depart, but neither the 27 ships remaining, nor any from outside, challenged the minefield for the next nine months. Aircraft also mined other harbors, river entrances, and areas along the coast--against none of which could the North Vietnamese mount a serious minesweeping effort. The mines did not need to sink ships to be effective, since traffic through deep water ports fell from forty ships per month to zero. The mines were "on duty" twenty-four hours a day without unduly exposing friendly forces to hostile fire. Moreover, since mines were passive "weapons that wait," they facilitated a blockade without aggressive action toward the shipping of non-belligerent nations, which previously traded with North Vietnam. Ultimately, Navy aircraft laid 8,000 mines during the war, and by successfully blockading the harbors to external supplies; they helped thwart the 1972 Communist offensive.
Operation Pocket Money, the mining campaign against principal North Vietnamese ports, was launched on 09 May 1972. Early that morning, an EC-121 aircraft took off from Danang airfield to provide support for the mining operation. A short time later, Kitty Hawk launched 17 ordnance-delivering sorties against the Nam Dinh railroad siding as a diversionary air tactic. Poor weather, however, forced the planes to divert to secondary targets at Thanh and Phu Qui which were struck at 090840H and 090845H, Vietnam time, respectively. Coral Sea launched three A-6A and six A-7E aircraft loaded with mines and one EKA-3B in support of the mining operation directed against the outer approaches to Haiphong Harbor. The mining aircraft departed the vicinity of Coral Sea at 090840H in order to execute the mining at precisely 090900H to coincide with the President's public announcement in Washington that mines had been seeded. The A-6 flight led by the CAG, Commander Roger Sheets, was composed of USMC aircraft from VMA-224 and headed for the inner channel. The A-7Es, led by Commander Len Giuliani and made up of aircraft from VA-94 and VA-22, were designated to mine the outer segment of the channel. Each aircraft carried four MK 52-2 mines. Captain William Carr, USMC, the bombardier/navigator in the lead plane established the critical attack azimuth and timed the mine releases. The first mine was dropped at 090859H and the last of the field of 36 mines at 090901H. Twelve mines were placed in the inner segment and the remaining 24 in the outer segment.With this mine blockade and other bombing in support of the South Vietnamese forces, the North Vietnamese advance was halted, and peace talks resumed in July 1972.
All MK 52-2 mines were set with 72-hour arming delays, thus permitting merchant ships time for departure or a change in destination consistent with the President's public warning. It was the beginning of a mining campaign that planted over 11,000 MK 36 type destructor and 108 special MK 52-2 mines over the next eight months. It is considered to have played a significant role in bringing about an eventual peace arrangement, particularly since it so hampered the enemy's ability to continue receiving war supplies.
On 11 May Naval aircraft flying from Coral Sea, Midway, Kitty Hawk and Constellation laid additional mine fields in the remaining ports of significance in NVN-- Thanh Hoa, Dong Hoi Vinh, Hon Gai, Quang Khe and Cam Pha as well as the Haiphong approaches. This early mining was not confined solely to the seven principal ports. Other locations, such as the Cua Sot, Cap Mui Ron, and the river mouths, Cua Day and Cua Lac Giang, south of Don Son and the Haiphong port complex, were also seeded early in the campaign.
The concept of aerial mining lives on, including through use of stealth bombers. See here where the nearby illustration came from. More info here.
UPDATES: Fixed the link to Maj. Chilstrom's article and offer his words up concerning the river mining in Vietnam mentioned by XBradTC in the comments:
As the war dragged on, President Lyndon Johnson escalated military action against North Vietnam, and in February 1967 allowed the Navy to mine five inland waterways (the Song Ca, Giang. Song Ma, Kien, and Cua Sot Rivers). The bombing campaign of Rolling Thunder begun in March 1965, had put pressure on the road and rail system and seemingly increased the importance of movement by barge and sampan. By April 1967, carrier-based A-6 aircraft laid the river minefields and caused the enemy to shift water-borne supplies back to already heavily burdened truck transport over jungle roadways at night. Still, the most lucrative targets--three main deep water ports (Haiphong, Hon Gai, and Cam Pha) remained off-limits to mines for another five years.