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A brilliant synergy between air and sea came close to defeating Japan even before the atomic bombs.

Late on the afternoon of March 27, 1945, 102 four-engine Boeing B-29s clattered aloft from Tinian, rising ponderously from the island and climbing slowly away, bound for another night raid on Japan. Since late November 1944, the Superfortresses had been carrying out deadly strikes against Japan’s increasingly defenseless cities, flying in hundred-plus ship formations night after night to pour a thousand tons of incendiaries at a time on Tokyo and other major Japanese towns. On March 9, nearly three hundred B-29s had struck Tokyo with devastating effect, burning sixteen square miles of the city—roughly a quarter of its entire area.

Yet this raid was to be a radical departure for the American bomber force. Signaling that this was no ordinary mission, Brig. Gen. John H. Davies, commander of the 313th Bombardment Wing, was at the controls of the first plane, personally leading the bomb groups into the western sky. In almost complete defiance of accepted air power dogma—not to mention top Army Air Forces brass back in Washington and the interservice boundaries they zealously patrolled— each of Davies’s B-29s was carrying a dozen huge sea mines instead of its usual load of hundreds of incendiary bombs. Their target was not the cities, but the lifelines that fed them: Japan’s sea-lanes. The mode of attack was not the direct application of overwhelming force against the enemy heartland that air power advocates had envisioned as the quintessential role for the strategic bomber, but was instead indirect, stealthy—and uncertain of success.

But the navy and a few quixotic enthusiasts within the air force were sold on the idea of aerial mining, and after years of closed doors, they had found an unexpected ally: legendary, cigar-chomping, no-nonsense Gen. Curtis E. LeMay, who had just taken charge of the B-29 force in January 1945.

By the end of the war, LeMay’s gamble would pay off handsomely in what is still one of the least-known chapters of American air power in World War II. Mines dropped by B-29s in Operation Starvation over the next few months would virtually shut down Japan’s crucial oil imports and its vital coastal shipping of coal and food. While debates rage to this day over the effectiveness of American fire raids on Japan’s cities in hastening the collapse of Japanese military power and helping to induce the surrender that finally came with the atomic bombings of Hiroshima and Nagasaki, there is little doubt as to the effectiveness of the mining campaign. Barely one month into the effort Japanese imports had fallen by 50 percent; by the end of the war, the air-delivered mines had sunk close to five hundred ships and damaged as many more, and imports were down an astonishing 90 percent. It was a fatal blow for a nation already severely strained by shortages and the continuing demand for food, fuel, and raw materials. As historian Kenneth Werrell notes, “Mining the waters around the home islands struck the fatal blow of the blockade, and the B-29s visible in the sky every day and the smoldering cities convinced the Japanese that they had no alternative but to surrender.”

The idea of using aircraft to place mines at long distance, stealthily, and without the risk that a foray by a surface ship would entail had actually been around for several years. It was another “bomber baron”—the Royal Air Force’s Arthur T. Harris—who first defied the stereo- types in pressing this unconventional concept against conventional thinking and service boundaries. Though remembered today for his fanatical pursuit of large-scale bombing of German urban and industrial areas—an advocacy that earned him the nickname “Bomber”—he was also an enthusiastic proponent of maritime air attack. Before the war, as an Air Ministry staff officer, he had successfully championed development and production of airdropped magnetic sea mines. Thanks to him, when war broke out, the RAF already had a hidden means of inflicting sea denial, and swiftly used it: in 1940, during the otherwise ill-fated Norwegian campaign, mines dropped by Hampden bombers sank a dozen ships; by the end of the year RAF mining had sunk eighty-two ships and damaged a further fifty-five.

RAF bombers took an impressive toll on Axis shipping through direct bombing attacks as well; overall, more than half of the surface ships and U-boats sunk by the Allies were accounted for by strikes from the air. But sortie per sortie, the air-carried sea mines dropped at night by RAF Coastal and Bomber Commands proved to be more than five times as effective; it took approximately 25 mine-dropping sorties to sink one ship, compared to 150 bombing sorties.

Much as the bomber offensive against Germany forced the diversion of large numbers of Luftwaffe personnel to defend the Reich, the mining campaign forced the Kriegsmarine to use 40 percent of its personnel simply to clear mines and the Luftwaffe to divert its fighter forces to coastal patrols. Mining effectively held Germany’s battle fleet in check throughout much of the war. Mine damage forced lengthy repairs to the battle cruisers Scharnhorst and Gneisenau, keeping them in port for much of their service. And mining shut the Baltic Sea to the German navy by September 1944, rendering it unusable for training, U-boat transit, and coastal shipping.

Despite this obvious success story, mining operations figured little in the planning of American sailors and airmen at the Pacific war’s outset. Both were wedded to a different kind of warfare: direct attacks on enemy ships at sea by bombs, torpedoes, and gunfire; or, in the case of the army’s airmen, by longrange strategic bombing against the Japanese heartland.

Such views frustrated mine advocates, none more than Lt. Comdr. Ellis Johnson. He was not a career military officer, but rather what the British would call a “boffin”—a scientific expert. Born in Quincy, Massachusetts, he earned a doctorate in electrical engineering from the Massachusetts Institute of Technology, and later, while working at Washington’s Carnegie Institution, he became a consultant to the Naval Ordnance Laboratory. As a specialist in magnetism, he was assigned to study mines and mine countermeasures. December 7, 1941, found him in the field on a consulting trip to discuss measures to render American ships safe from magnetic mines through the method known as “degaussing”: applying an electromagnet to counter the ship’s magnetic field. The place was the Pacific Fleet—at Pearl Harbor.

In the carnage that followed, Johnson helped get surviving ships out of harbor. He then returned to Washington, fired with the idea of launching a mine campaign against Japan, accepted a commission, and returned to the Pacific to make his dream a reality. It would take far longer than he could have imagined.

More than one strategist had noted Japan’s unusual vulnerability to mining attacks. In December 1942, retired admiral William V. Pratt wrote in Newsweek that Japanese seamen transited innately “hazardous waters,” and that “almost every mile of this water is mineable.” Japan imported 80 percent of its oil, 88 percent of its iron, and 20 percent of its food supplies. Coal was particularly crucial, for fully half the power used by Japanese industry came from coal shipped via coastal waters. Overall, three-quarters of all Japanese transportation involved coastal traffic. Each month, 520,000 tons of ship traffic passed through the Shimonoseki Straits, including all-important supply ships to and from China and Korea.

The very first Japanese ship to fall prey to mines met its fate just a week before Pratt’s Newsweek article appeared. As Lt. Comdr. Roy Benson, skipper of the submarine USS Trigger, peered through his periscope, he saw an 8,000-ton cargo vessel pass over a minefield the Trigger had just laid off of Tokyo. The ship blew up and swiftly sank below the waves.

But submarines were hardly the most efficient vehicles for transporting mines that weighed up to a ton apiece. Final statistics from the war would ultimately show that once Ellis Johnson and others broke down the institutional resistance to aerial mining, aircraft were responsible for nearly 90 percent (more than 21,000) of the 25,000 Allied mines laid in Japanese waters throughout the entire war, compared to the 3,000 mines delivered by surface ships, and the bare 650 delivered by submarines.

Aerial mining was a natural partnership for the air force and navy. As a postwar Army Air Forces study noted, “The Army had the aircraft [and] the Navy had the mines.” But getting the two to cooperate was another matter. By 1944, Johnson’s wartime experiences had convinced him that the best means of seeding Japanese waters was with long-range strategic bombers; experiments with carrier aircraft had led to high losses for little gain, and even the navy’s ubiquitous PBY Catalina flying boats took higher than acceptable losses.

On the other hand, the Army Air Forces’ cooperation with the RAF and Royal Australian Air Force in initiating a mining campaign against Japanese-controlled territory in South Asian waters in 1943 provided definitive proof of the effectiveness of long-range bombers in this role. On February 22, 1943, long-range four-engine B-24 Liberators of the India-based Tenth Air Force dropped the first aerial mines used against Japanese targets, planting forty in the Rangoon River. The Rangoon seeding marked the beginning of an intensive port-and-river closing campaign in the theater. Repeated mine seedings of key ports achieved startling effects. By the end of 1943, Rangoon was shut down; similar intensive treatment effectively closed Balikpapan, Surabaya, Moulmein, Bangkok, Singapore, Haiphong, and Hong Kong. Attackers even sowed rivers well inland; so densely did they seed China’s Yangtze River with floating magnetic mines that nearly fifty Japanese ships were sunk or damaged, achieving, as a subsequent Navy study concluded, “a substantially complete blockade.” Confronting the collapse of their logistical lines, the Japanese switched to using smaller, less capable wooden-hulled ships or (where possible) inefficient land transport; neither worked.

The B-24, as useful as it had been in these attacks, had great range but lacked the survivability to attack the Japanese homeland. Successfully mining Japan would require using the Army Air Forces’ new long-range four-engine Boeing B-29 Superfortress: no other airplane had the range, capacity, and survivability to undertake the task.

Introduced into combat in June 1944, the B-29 had so far proven a disappointment, plagued by a variety of problems. And Johnson—now working for Adm. Chester Nimitz, commander in chief of the Pacific Ocean Areas—had briefed his mining plan to the leadership of the Army Air Forces’ XXI Bomber Command in Hawaii in early July 1944, only to be met with a tepid response. Focused on pulverizing Japan’s industry and military machine, most Army Air Forces senior leaders considered mining at best an undesirable distraction. Their reluctance caused Nimitz to write directly to Gen. Henry H. “Hap” Arnold, chief of the Army Air Forces, enumerating the benefits that could accrue from mining and requesting use of the big planes in an aggressive campaign.

Arnold reluctantly agreed, though on a delayed schedule and with a smaller effort than Nimitz desired. Nevertheless, having surmounted this hurdle, joint planning went ahead, with mine experts and airmen coming together to structure the campaign. Now LeMay’s enthusiasm took hold. Frustrated by the troubles the B-29 had experienced with its early operations, LeMay was receptive for new ways to use the bomber to maximum effect. Like Bomber Harris, LeMay was not only sold on the idea but had the same hard-charging personality needed to get past institutional differences and get things done. He made his own changes to the mining plan—reversing his boss’s orders to return the plan closer to what Johnson and Nimitz had envisaged. Afterwards, Johnson recalled: “It was the firm belief in and support of the mining effort by General LeMay himself that made the successful campaign possible at all.” Johnson, Nimitz, and LeMay all recognized that Japan had already suffered fearful depredation of its merchant fleet. Over the length of the war, it had declined more than 80 percent, to less than 1.5 million gross tons. Any further reduction in shipping would have disastrous consequences for Japan’s war effort and people.

The synergistic weapons to achieve this strangulation were potentially at hand with the Army Air Forces’ Boeing B-29 and the navy’s sea mines, though both had their problems. Powered by four Wright R-3350 engines producing 2,200 hp apiece, Boeing’s graceful B-29 Superfortress lived up to its name. Its broad, low-drag high-lift wings spanned 141 feet, and it had a maximum takeoff weight of 135,000 lb. Highly streamlined, with the ability to fly 357 mph at 30,000 feet (a challenge for most Japanese fighters), it could carry a 10,000-lb bomb load over a range of 3,250 miles. Its crew of eleven included a bombardier with a radar bombsight and four gunners who used sophisticated remote-sighting stations to control the plane’s defensive gun turrets. The most complex airplane of its time, its development consumed $3 billion (roughly $33 billion in today’s money), making it even more expensive than the atomic bomb.

But for all this, the Superfortress had a troubled history, plagued particularly by engine problems and manufacturing difficulties that delayed its entry into service. Engines caught on fire with terrifying frequency, for one thing. Nevertheless, by the end of March 1945, the Army Air Forces was accepting over three hundred of the big bombers each month from Boeing and its subcontractors, and the XX and XXI Bomber Commands were flying over 3,100 sorties per month against Japanese targets, smothering them in nearly 15,300 tons of bombs.

The navy had 1,000- and 2,000-lb mines that could be used in the Starvation campaign, with a variety of different fuse types: the magnetically detonated M-9 Mod 1 and M-11; the acoustically triggered A-3 (set off by propeller noise) and A-5 (by ship hull noise); and the pressure-sensitive A-6, which exploded when it experienced a reduction in hydrostatic pressure characteristic of a ship passing directly overhead. Of these, however, only the M-9, M-11, and A-3 were immediately available for use by the B-29 force. And, as ill chance would have it, the Japanese had captured examples of all of them.

Thus, all had to have their detonation mechanisms changed to defeat possible Japanese countermeasures. But the navy’s mine experts swiftly set to work in Hawaii and at a new Mine Assembly Depot No. 4 on Tinian. By late February 1945, a stock of 1,500 modified mines was ready for use.

Air force brass selected the highly experienced 313th Bombardment Wing, equipped with the new AN/APQ-13 attack radar, to develop the operational plan and execute the strikes. Strikes would be made at night by aircraft flying independently at spaced intervals to the target and dropping mines on parachutes from an altitude of five thousand to eight thousand feet. The bombardier and navigator would use radar bearings of a prominent portion of nearby coastline to place the mines in exact spots crossing Japanese ship channels; they would also need to calculate precisely the effect of wind in causing the mines to drift during their descent. The 313th began a series of rigorous practice missions, training in the use of the radar and dropping both water-filled dummy bombs rigged with an adapter to simulate the trajectory of a falling mine and real 1,000-lb mines.

At LeMay’s insistence, to achieve maximum shock effect, the first seedings would be at full wing strength against the Shimonoseki Straits. Led by General Davies, Operation Starvation first took to the air on March 27, 1945, and lasted through August 15, six days after the dropping of the second atomic bomb. Starvation involved five sequential phases:

Phase I (March 27–May 2) supported the Okinawa invasion by closing off the Shimonoseki Straits.

Phase II (May 3–May 12) squeezed industrial centers by mining Inland Sea routes, Kobe, Osaka, and Tokyo, and remining the Shimonoseki Straits.

Phase III (May 13–June 6) mined northwest Honshu and Kyushu ports.

Phase IV (June 7–July 8) intensified mining of Honshu and Kyushu ports.

Phase V (July 9–August 15) continued remining, extended mining to Korean targets, and closed Moji and Niigata ports.

Army Air Forces bombers dropped over 12,000 mines during Starvation, creating densely seeded fields that averaged nearly 470 mines apiece. The effects were almost instantaneous. Imports plunged. In early April 1945, with the more convenient Shimonoseki Straits closed, the Japanese superbattleship Yamato was forced to sortie via the Bungo Strait on its ill-fated mission to Okinawa. There it was spotted by American aircraft, attacked by carrier planes, and sunk.

Accompanied by leaflet drops, the mining had a profound psychological impact as well. One Japanese officer, Comdr. Tadao Kuwahara, later told Allied investigators: “It was difficult to obtain crews towards the end of the war….There was a great tendency for crews to jump ship.” Although vigorously conducted, Japanese sweeping and other defensive countermeasures—even using suicide boats to detonate mines—proved unequal to the task of keeping sea-lanes open. Traffic through the Shimonoseki Straits fell to 10 percent of its normal rate by June 1945, by which time other Japanese ports, plus crucial Korean ports controlled by Japan, were either closed or useless—including Moji, Niigata, Fusan, and Rashin. After the war, Capt. Kyuzo Tamura, a minesweeping expert, conceded, “The result of B-29 mining was so effective against the shipping that it eventually starved the country,” adding that it was “a far-sighted policy.”

Nimitz wasted no time in praising the XXI Bomber Command’s airmen for their “precision and determination,” assuring them they had made “a definite contribution toward winning the war,” and praising their “phenomenal results.” Indeed so, though for the Superfortress crews, such accomplishment did not come without risk and loss. The flying challenges—weather, navigation, mechanical reliability—were bad enough: an average mission, with twelve 1,000-lb or seven 2,000-lb mines, covered 2,870 miles, and raids against Korea could reach 3,675 miles, necessitating a fuel stop at Iwo Jima. Then there was the enemy: on the first night of the Starvation strikes, three B-29s were lost over the Shimonoseki Straits, and another eight damaged. Searchlights caught some of the bombers, and flak swept the skies. “The inside of the plane,” radio operator Park Lovell recalled of one encounter, “was like being in a welding booth”; his pilot dived out of the accusing lights to safety, and Lovell said he “became a little more religious at that time.”

On another mission, the B-29’s notorious propensity for engine fires asserted itself. A 9th Bomb Group Superfortress experienced an engine fire right at takeoff, forcing an emergency jettisoning of the mines. Two of the mines collided on descent and exploded, the blast nearly removing the tail of the plane, and fragments seriously wounding a gunner. Though the pilots returned it safely to earth, the Superfortress never flew again.

Overall, combat losses and accidents during Starvation cost the lives of 103 airmen and fifteen aircraft. Yet aerial mining of Japan’s home waters had sunk or damaged 961 ships totaling 2,000,000 tons. Of these, aerial mines sunk 484 ships representing nearly 650,000 tons. Ship passage through the Shimonoseki Straits decreased from 70,000 tons per day in March to 35,000 tons in April, plummeted to 7,500 tons in May and 1,750 tons in June, staggered up to 8,000 tons in July, and dropped to 1,500 tons in August. B-29 mining caused 60 percent of Japanese shipping losses between March and August 1945. Further, nearly 90 percent of Japan’s twenty-two repair yards were mine-blockaded, stranding damaged vessels.

As Allied forces came ever closer to the Japanese homeland, air became increasingly dominant in the sea war. As former submarine skipper Rear Adm. Ignatius “Pete” Galantin recalled, “It had become an aviator’s, not a submariner’s war.” Of the 143 Japanese ships sunk by bombs, torpedoes, and mines in July 1945, American submariners accounted for less than 10 percent; airmen accounted for over 90 percent, fully 129 vessels.

Aerial mining had proved itself to risk fewer lives, consume fewer resources, and, proportionally, achieve far more with less than had the much-heralded submarine campaign. Counting only sinkings, B-29 mining claimed a monthly average of 63 ships in Japanese home waters, while submarine attacks (using both mines and torpedoes) claimed a monthly average of 30. The postwar Joint Army-Navy Assessment Committee credits 190 American submarines with sinking 1,314 ships, thus averaging 6.92 ships sunk per submarine. But the B-29 mining force deployed against Japan averaged only 40 aircraft, yielding an average of 7.08 ships sunk per B-29.

Thus, a B-29 with a crew of eleven men achieved roughly the same results as a fleet submarine with a crew of eighty-five. Furthermore, the United States spent only $90,000 in B-29 acquisition costs per Japanese ship sunk versus $500,000 in submarine acquisition costs. Additionally, while the mining aircraft dropped a relatively “stupid” and reasonably cheap weapon, the submarine fired a far more expensive miniature submersible, the self-propelled torpedo. Although fewer torpedoes were expended per sunken Japanese ship than mines (eleven torpedoes vs. forty-three mines), mines were clearly cheaper and, more important, did not require the dangerously close approach necessitated by the torpedo.

Looking back at the heroic accomplishments of the B-29 aerial miners, one can easily comprehend why the analysts of the postwar United States Strategic Bombing Survey wrote, “It is believed that this campaign, begun earlier and laid on with greater weight, would have reduced effective shipping nearly to the vanishing point. It would have produced a condition of crisis in Japan sooner than actually occurred.”

Though the shock of the atomic bombs undoubtedly gave Japan’s military and political leaders the rationale they needed to force the hand of militants opposed to surrender, the words of Japanese premier Kantaro Suzuki are worth recalling: “It seemed to me unavoidable that in the long run Japan would be almost destroyed by air attack so that merely on the basis of the B-29s alone I was convinced that Japan should sue for peace.” Though the least known of the B-29’s conventional missions, the delivery of aerial mines was, pound for pound, the most potent part of that inexorable destructive power.


Originally published in the May 2008 issue of World War II Magazine. To subscribe, click here