Late on the afternoon of 27 March 1945, 102 Boeing B-29 Superfortresses lifted off from Tinian. The crews had been firebombing Japanese cities for weeks, but on this night their bomb bays carried no incendiaries. Each bomber held a dozen sea mines.

Their target was not a factory or a city but a narrow stretch of water: the Shimonoseki Strait, the channel between Honshu and Kyushu a vital route for Japanese shipping. It was the opening night of Operation Starvation, the US Army Air Forces campaign to strangle Japan from the air by sowing its harbours and sea lanes with mines.

Over the next five months the B-29s laid about 12,000 mines. Long overshadowed by the fire raids and the atomic bombs, it has been called the most efficient means of destroying Japanese shipping in the whole war.

A Scientist's Idea Nobody Wanted

The champion of aerial mining was not a career officer. Lieutenant Commander Ellis Johnson was a scientist, a specialist in magnetism who had become a consultant to the Naval Ordnance Laboratory. Mines laid from the air had already closed ports across Southeast Asia, many of them dropped by B-24 Liberators, but Japan's home waters needed an aircraft with far more range and survivability.

Only the B-29 had it. Working for Admiral Chester Nimitz, commander in chief of the Pacific Ocean Areas, Johnson briefed a mining plan to XXI Bomber Command in Hawaii in early July 1944 and met a tepid response. Most Army Air Forces leaders saw mining as a distraction from strategic bombing, so Nimitz wrote directly to General Henry "Hap" Arnold to ask for the big bombers.

Arnold agreed reluctantly, on a delayed schedule and with a smaller effort than Nimitz wanted, according to historian Richard P. Hallion. He handed the job to General Curtis LeMay, who had taken over the B-29 force in January 1945. LeMay, keen to find better ways to use his troubled bomber, backed the plan and insisted that the first seeding be flown at full wing strength against Shimonoseki.

A wartime US Navy film on aerial mine warfare: how mines were armed, fused and laid from aircraft, including footage of PBJ mining missions in the Pacific.

The First Night Over Shimonoseki

The job went to the 313th Bombardment Wing, whose B-29s carried the AN/APQ-13 radar. Crews flew to the target individually at night, released parachute-retarded mines from about 5,000 to 8,000 feet, and used radar bearings on the coastline to place each mine across a shipping channel. Navy specialists at Mine Assembly Depot No. 4 on Tinian had about 1,500 modified mines ready by late February.

Brigadier General John H. Davies, the wing commander, flew the lead aircraft on 27 March. Pacific Wrecks records that 92 of the 102 B-29s mined the Shimonoseki Strait that night. Searchlights and flak were waiting, and Hallion counts three B-29s lost over the strait and eight more damaged.

The mines themselves were influence weapons with magnetic and acoustic firing mechanisms, later joined by pressure types that fired on the change in water pressure from a passing hull. A typical load was twelve 1,000-lb or seven 2,000-lb mines, and an average round trip covered 2,870 miles.

Five Phases, 12,000 Mines

Hallion divides Starvation into five phases. The first, from 27 March to 2 May, supported the invasion of Okinawa by closing the Shimonoseki Strait. Later phases mined the Inland Sea, Kobe, Osaka and Tokyo, then the ports of northwest Honshu and Kyushu, and finally reached the Korean ports before the campaign ended on 15 August.

The effect on traffic was brutal. According to Hallion, tonnage through the Shimonoseki Strait fell from about 70,000 tons a day in March to 7,500 in May and 1,750 in June. Wikipedia, citing Ronald Spector, records that shipping through Kobe dropped by 85 percent between March and July, and that 35 of 47 essential convoy routes had to be abandoned.

Japanese minesweepers, and even suicide boats sent to set off the mines, could not keep the sea lanes open, Hallion writes. The US Strategic Bombing Survey, reporting in July 1946, found that the B-29 mines had become the leading killer of Japanese ships in the last months of the war.

How Many Ships? It Depends Who Counted

The totals are not settled, and the sources disagree. Wikipedia, citing a 1978 US Naval Institute Proceedings article, records 1,533 sorties, 12,135 mines laid in 26 fields on 46 missions, and 670 ships sunk or damaged, totalling more than 1,250,000 tons. The Strategic Bombing Survey puts the number of mines sown at 12,034.

Hallion gives a higher figure: 961 ships sunk or damaged, totalling about 2,000,000 tons, of which 484 ships of nearly 650,000 tons were sunk outright. The price was modest by the standards of the B-29 war. Wikipedia lists 16 B-29s lost, including three scrapped after battle damage, while Hallion counts 15 aircraft and 103 airmen killed.

Mining also took only a small share of the bomber force. According to Wikipedia, it accounted for 5.9 percent of XXI Bomber Command's successful sorties. Hallion's comparison is striking: on average, each mining B-29 sank about as many ships as a US fleet submarine, at a fraction of the cost.

The Verdict: Too Little, Too Late?

The Japanese felt the squeeze at once. Hallion quotes Commander Tadao Kuwahara telling Allied investigators that it became difficult to find crews and that sailors tended to jump ship.

By June 1945, Hallion writes, traffic through the Shimonoseki Strait had fallen to 10 percent of its normal rate, and ports such as Moji and Niigata, along with Japanese-held Korean ports, were closed or useless. Captain Kyuzo Tamura, a minesweeping expert, was blunter after the war.

The Strategic Bombing Survey reached a similar conclusion from the other side. Its analysts argued that much duplicated effort in the final assault on Japan could have been avoided, and that the strangulation of the Japanese economy could have been pushed harder and sooner.

Its prescription was specific: start the aerial mining program earlier, aim carrier strikes at the remaining merchant fleet rather than at warships already immobilised, and hit Japan's fragile railway network.

That is the uncomfortable lesson of Operation Starvation. The B-29s that burned Tokyo in the Operation Meetinghouse raid were the same type that quietly closed Japan's sea lanes. One campaign made headlines; the other did its work out of sight.

Mining also asked crews to fly long, lonely night sorties for results they would never see. A raid on the Korean ports could stretch to 3,675 miles and needed a fuel stop at Iwo Jima.

The Last Bomb (1945), the Oscar-nominated wartime documentary on the conventional B-29 campaign against Japan, opening on the bomber bases of Saipan, Guam and Tinian.

Japan's cities burned in public. Its harbours died in the dark, one parachute at a time, and by August the island nation that lived by the sea could barely use it.

Sources: Wikipedia (Operation Starvation), US Strategic Bombing Survey Summary Report (Pacific War, 1 July 1946), Richard P. Hallion, "Operation Starvation", HistoryNet / World War II Magazine, Pacific Wrecks (Shimonoseki missions)