Honolulu, Hawaii · October 29, 1943

In World War II’s Pacific Theater, sea mines—explosive underwater devices that damaged, sank, or deter­red Japa­nese war­ships, sub­marines, and mari­time com­merce—were wea­pons that had dif­ficulty gaining the same respect as guns, bombs, and tor­pe­does enjoyed in the U.S. ar­senal. Over time, how­ever, a small number of mining advo­cates in both the U.S. Navy and the U.S. Army Air Forces in­fluenced their ser­vice bosses enough to en­sure the growth of offen­sive mine-laying, equip­ment develop­ment, and com­bat experi­ence. On this date in 1943 U.S. sub­ma­rines began mining the waters off French Indo­china. The following March the U.S. Navy mounted a direct aerial mining attack on Japa­nese ship­ping on Palau Island in the west­ern Pacific, which stopped 32 Japa­nese ships from escaping Palau’s har­bor. Com­bined with bombing and strafing attacks, the opera­tion sank or damaged 36 ships. The most suc­cess­ful mining opera­tions were those con­ducted by the Allied air forces laying aerial mine­fields. Begin­ning with a very suc­cess­ful attack on the Yan­gon River in Burma (Myan­mar) in Febru­ary 1943, B‑24 Libera­tors, PBY Cata­linas, and other avail­able bomber air­craft took part in local­ized mining opera­tions in the China Burma India (CBI) Theater and in the South­west Pacific (Philip­pines, the Dutch East Indies, Bor­neo, New Gui­nea, and the west­ern Solo­mon Islands). British and Royal Aus­tralian air forces carried out 60 per­cent of the sor­ties and the U.S. Army Air Forces and U.S. Navy carried out the balance. U.S. Adm. Thomas C. Kinkaid, who directed nearly all RAAF mining opera­tions in the CBI, wrote in July 1944 that “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.” The U.S. mining effort against the Japa­nese home islands proved very suc­cess­ful, closing major ports like Hiro­shima on west­ern Honshū, the largest home island, for days. At best, the Japa­nese suc­ceeded in sweeping only about 50 per­cent of Amer­i­can acoustic mines (they meas­ured sound of cer­tain fre­quen­cies). Pres­sure mines, the most com­monly used against Japan near the end of the war, were even more dif­fi­cult to sweep. By war’s end, more than 25,000 U.S.-laid sea mines were still in place. Over the next 30 years, more than 500 mine­sweepers were damaged or sunk in continuing clearance efforts.

Operation Starvation: Strangling Japanese Maritime Traffic

B-29 aerial mining M26 sea mine

Left: Overseen by Maj. Gen. Curtis LeMay, Operation Star­va­tion was a joint U.S. air and naval effort to strangle Japa­nese mari­time traffic by the aerial mining of Japan’s har­bors and straits. The main objec­tives of Opera­tion Star­va­tion were to pre­vent the impor­ta­tion of raw mate­rials and food into Japan, pre­vent the supply and move­ment of mili­tary forces, and disrupt shipping in the Inland Sea. Beginning on March 27, 1945, B-29 Super­for­tresses assigned to Opera­tion Star­va­tion dropped 825 parachute-retarded influ­ence mines with mag­ne­tic and acous­tic explo­ders. The ini­tial sortie was followed up on March 30 by 1,528 more. Some models of mines had water-pressure-displace­ment explo­ders. Aerial mining proved the most effi­cient means of destroying Japa­nese shipping during the war. In terms of damage per unit of cost, it surpassed the stra­te­gic bombing and the U.S. submarine campaigns against Japan.

Right: A 1,000 lb Mk 26 sea mine being dropped by a B‑29, 1945. LeMay’s XXI Bomber Com­mand (a unit of the Twentieth Air Force in the Mariana Islands) laid 12,135 mines in 26 fields on 46 sepa­rate mis­sions. The Japa­nese employed 349 vessels and 20,000 men to clear mines. Over the course of the war, aerial, sur­face, and sub­marine mine-laying sank or damaged over 2 million tons of enemy ship­ping, a volume repre­senting nearly one quarter of the pre­war strength of the Japa­nese mer­chant marine. After the war, the com­man­der of Japan’s mine-­sweeping opera­tions noted that he thought the U.S. mining cam­paign could have led directly to the defeat of Japan on its own had it begun earlier.

Sea mines ready for loading MK 25 sea mine being loaded into B-29

Left: Sea mines prepared by mine assembly personnel on Tinian Island in the Northern Marianas are ready for loading onto B‑29s.

Right: A 2000 lb MK 25 mine is loaded into a B‑29’s bomb bay. Air­craft were loaded with an average of 12,000 lb of mines con­sisting of a mixture of 2,000 lb MK 25 mines and 1,000 lb MK 26 mines. A mix of magne­tic and acous­tic actu­ating devices were used with various sensi­tivity settings, a ran­dom mix of arming delays between 1 and 30 days, and ship counts between 1 and 9.

1945 U.S. Army Air Forces Strategic Bombing of Japan