Nuclear attacks against Japan

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Nuclear weapons have never been used except twice by the United States against the Japanese at the very end of the Second World War in August, 1945. They were developed in a highly secret wartime program, the Manhattan Project during the war. That Project dealt with the technology, not initially policy or targeting, and did much of its work expecting the weapons to be used against Germany. Weapons were ready, however, only after Germany surrendered.

Nuclear weapons were simply not part of the overall U.S. strategic planning process, but outside it. President Franklin D. Roosevelt was first briefed on an actual bomb production schedule in December 1944, indicating the first bomb could be ready in August 1945. The War Department Operations Division (OPD) took no role in planning; there were no references to nuclear weapons in its files until 6 August 1945, although a few senior planners had some knowledge of it. [1]. Secretary of War Henry Stimson was not thoroughly informed of the successful Trinity test of a nuclear weapon until July 21. [2]

The decision was controversial at the time, with the decisio nmakers knowing less than we do today, both on the effects of nuclear weapons, and on the internal Japanese arguments about conditions under which they would surrender. Further, the decision was made largely outside the regular strategic process, due to the combination of secrecy about development and uncertainty if the bomb would work. President Harry S Truman was unaware of the Manhattan Project until after he became President, and was briefed on 25 April. At least wwo groups, the Interim Committee of civilian advisers and a scientific advisory panel, recommended use of nuclear weapons. The orders were issued by the Joint Chiefs of Staff. Pacific theater commanders Chester W. Nimitz and Douglas MacArthur were informed of the decision rather than consulted, although strategic air warfare against Japan was also outside their control; the JCS commanded it directly through the Twentieth Air Force.

It has been suggested that the U.S. Strategic Bombing Survey said the Japanese would have surrendered without the use of nuclear weapons, but the actual report emphasizes that opinion is made with the benefit of hindsight. The Report said:

Certain of the United States commanders and the representatives of the Survey who were called back from their investigations in Germany in early June 1945 for consultation stated their belief that, by the coordinated impact of blockade and direct air attack, Japan could be forced to surrender without invasion. The controlling opinion, however, was that any estimate of the effects of bombing on the Japanese social fabric and on the political decisions of those in control of Japan was bound to be so uncertain that target selection could safely be made only on the assumption that ground force invasion would be necessary to force capitulation.[3]

The Survey continued,

With the benefit of hindsight, it appears that the twin objectives of surrender without invasion and reduction of Japan's capacity and will to resist an invasion, should the first not succeed, called for basically the same type of attack. Japan had been critically wounded by military defeats, destruction of the bulk of her merchant fleet, and almost complete blockade. The proper target, after an initial attack on aircraft engine plants, either to bring overwhelming pressure on her to surrender, or to reduce her capability of resisting invasion, was the basic economic and social fabric of the country. Disruption of her railroad and transportation system by daylight attacks, coupled with destruction of her cities by night and bad weather attacks, would have applied maximum pressure in support of either aim.

Some "revisionists" have suggested Hiroshima was supposed to be an unmistakable signal to Joseph Stalin|Stalin to play along diplomatically with the Americans who planned to rule the postwar world. Many have asked whether some sort of demonstration explosion should have been made, in order to frighten Tokyo without killing so many people. The option was considered, but with only two bombs available Truman decided instead to drop millions of leaflets upon Hiroshima and Nagasaki warning people to leave immediately, and at the Potsdam Conference he explicitly warned Japan it must surrender immediately or be hit with terrible force.

The civilian government in Tokyo wanted peace on conditional terms, but that was impossible because of Roosevelt's policy of unconditional surrender (Second World War), and because the civilians did not control Japan's decisions. Only the unprecedented direct intervention of the Emperor changed the balance of power.

Many factors led to the surrender of Japan. The nuclear bombings certainly were a part. The declaration of war by the Soviets, however, ended any hope for a negotiated peace. Even after Hiroshima and the invasion of Manchuria the Army and Navy wanted to fight on, while the civilians wanted to give up. With Roosevelt gone, the Americans redefined "unconditional" to allow continuance of the Emperor. Hirohito then broadcast an order to the nation and its armed forces to surrender, which was immediately obeyed.

Nuclear weapons development

For more information, see: Los Alamos National Laboratory and Manhattan Project.

President Harry S. Truman had been unaware of any specifics of the Project. While in the Senate, he had become aware of the extremely large expenditures in the project, but, after General George C. Marshall, a man of unquestioned integrity, told Truman it was a real, critical, and utterly secret project, Truman called off further investigations.

The first bomb to be used, a uranium fission device of the "gun" type code-named Little Boy (nuclear weapon)|Little Boy, had not been tested; only theoretical calculations of effect were available. Physicists involved in its development were certain it would work, but less so about the plutonium implosion technology in the second bomb. In the Trinity test in New Mexico, an implosion device of the type used on Nagasaki was tested, and better data was available.

The decision to use nuclear weapons

There was no consensus, among the small number of senior military leaders aware of the bomb development, about the separable issues of military effectiveness of such attacks, and the ethics thereof. While the casualties that would actually be caused by nuclear attacks were not known, the fire-bombing of Tokyo probably caused a greater number of casualties.[3]

From the Trinity test experience, however, it was clear that a nuclear explosion would be qualitatively different than any previous attack, and would have great psychological impact. Nevertheless, there were both military commanders, and scientists that worked on the bombs' development, that preferred such measures as an initial demonstration, for the Japanese, on an uninhabited target. Other commanders and scientists believed that the shock value of the weapons would contribute to the ending of the war. Further complicating the situation was that the new President, Harry Truman, had not been informed of the bomb development while Vice President, and had a short time to make the decision.

The U.S., in anticipation of a possible nuclear attack, had avoided heavy bombing of four cities, including Hiroshima and Nagasaki. This had two purposes: allowing better assessment of the weapon effects, and also having a greater shock value.

Effect of secrecy

Very few of Truman's advisers complete knowledge at the policy level: [4]

Individual Japanese positions Details of invasion Nuclear weapons
Henry Stimson Good Complete Complete
George C. Marshall Good Complete Complete
Joseph Grew Best Moderate None
William Leahy Good Complete Aware, but convinced they would not work
James Forrestal Good Complete Shallow
James Byrnes Shallow Shallow Shallow


The senior civilian panel was the Interim Committee, whose principal members were: Vannevar Bush, James Conant, Arthur Compton, Under Secretary of the Navy Ralph A. Bard, Assistant Secretary of State William L. Clayton, and James Byrnes. George C. Marshall representeded the military to it. It had two advisory panels, one from science and one from business. Henry Stimson, accompanied by Leslie Groves, briefed the President on its recommendation: The attack should take place as soon as possible and without warning.

Truman and Stimson then discussed the larger world situation with respect to nuclear weapons. They agreed not to provide details to the Soviets without concessions. Stimson explained that

the Interim Committee was considering domestic legislation and that its members generally held the position that international agreements should be made in which all nuclear research would be made public and a system of inspections would be devised. In case international agreements were not forthcoming, the United States should continue to produce as much fissionable material as possible to take advantage of its current position of superiority.[5]

Chaired by J. Robert Oppenheimer, with Ernest Lawrence, Enrico Fermi, and Arthur Compton as members, the Science Panel recommended "before the weapons are used not only Britain, but also Russia, France, and China be advised that we have made considerable progress in our work on atomic weapons." They continued to observe

The opinions of scientific colleagues on the initial use of these weapons are not unanimous; they range from the proposal of a purely technical demonstration to that of the military application best designed to induce surrender. Those who advocate a purely technical demonstration would wish to outlaw the use of atomic weapons, and have feared that if we use the weapons now our position in future negotiations will be prejudiced. Others emphasize the opportunity of saving American lives by immediate military use, and believe that such use will improve the international prospects, in that they are more concerned with the prevention of war than with the elimination of this specific weapon. We find ourselves closer to these latter views; we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use. [6]

The business panel members were Walter S. Carpenter of DuPont, James C. White of Tennessee Eastman, George H. Bucher of Westinghouse, and James A. Rafferty of Union Carbide.

Leo Szilard and 68 members of the staff of one of the Manhattan Project facilities, the Chicago Metallurgical Laboratory, petitioned the President on 17 July.

first, that you exercise your power as Commander-in-Chief to rule that the United States shall not resort to the use of atomic bombs in this war unless the terms which will be imposed upon Japan have been made public in detail and Japan knowing these terms has refused to surrender; second, that in such an event the question whether or not to use atomic bombs be decided by you in the light of the consideration presented in this petition as well as all the other moral responsibilities which are involved.[7]

Alternatives to nuclear attack

An argument against using these weapons was that Japan was clearly struggling under conventional bombings and the submarine blockade. Unfortunately, the U.S. had no sources inside the Japanese government, which would confirm that there was a stalemate between a hard-line faction that believed it appropriate to fight to the last Japanese, and a faction that was willing to examine a peace. The peace faction had come into significance with victory in the Battle of Saipan and the resultant fall of the Hideki Tojo|Tojo government, but the Allies had no hard information.

The primary argument was to use a radically different attack with the purpose of breaking the will of Japan, which actually was unlikely to affect the hard-liners. A secondary consideration was that the U.S. was planning a land invasion of Japan, with the first phase, Operation Olympic scheduled in October 1945, with the target of Kyushu, which was under the command of Second General Army. That organization, a major part of the Japanese Operation KETSU-GO defensive plan, had its headquarters in Hiroshima Castle.

In July 1945 the Army Air Force saw its doctrine of conventional strategic bombing working as intended. The original plan was to have used B-29 bombers, from high altitude, with greater precision than the B-17 and B-24 bombers used in Europe. Unexpected high-altitude winds proved this was impossible, and Gen. Curtis LeMay, newly commanding the strategic bombers, on his own authority changed to low-altitude incendiary bombing. He shed the machine guns and gunners, and the gasoline no longer needed to lift the planes to 30,000 feet. The result was a doubling of the bomb load, and very scared fliers who were greatly relieved to discover their losses were less using the new tactics.[8] From the first raid on March 9, the new tactic was devastating. working to perfection. The B-29 dropping conventional high explosives and incendiaries was the perfect instrument to destroy the infrastructure of Japan's larger cities. The great bombing campaign had just started; it was planned to peak a year later. The atomic bomb was not part of AAF doctrine; the AAF generals had not been consulted, knew very little about the bomb and even demanded a direct order from President Truman before they agreed to explode it.

The U.S. Navy had very little to do with the atomic bomb decision. It had gotten reluctant Air Force support to lay large mine (naval)|minefields in the inland waterways of the Home Islands. The blockade combined mines, submarine operation, air strikes from aircraft carriers, and some surface warships. It argued that the blockade was working well, cutting off nearly all oil, food and troop movements to and from Japan. It expected that the blockade would eventually lead to surrender.

The Army agreed that the combination of blockade and strategic bombing would eventually destroy every Japanese city, but felt it could not destroy the Japanese Army, which was widely dispersed and dug in.

Other uses for the weapons

For more information, see: Planned tactical nuclear attacks on Japan.

General Marshall worried that the American people might grow weary of more years of warfare, and might even demand some sort of compromise peace in order to bring the soldiers home. (Marshall underestimated the intense determination of nearly all Americans to destroy Japan.) Furthermore he objected to dropping the bombs on cities on moral and political grounds (Japan might become an enemy forever). Most of all, he had a tactical rather than strategic use in mind. Only a handful of bombs were being built, (two to four per month) and MacArthur's invasion forces ought to have all of them. Nine bombs had been allocated to Operation Olympic. Marshall and his planners concluded that Japan would surrender only after ground troops captured Tokyo. The invasion of Kyushu was scheduled November 1; all bombs available then should be used there. They would give invading infantry forces enough firepower to destroy defensive ground installations, communications facilities, kill exposed enemy soldiers, and also block the arrival of reinforcements. To waste the precious bombs on irrelevant civilians would cause more American casualties, and like all the high American officials Marshall was committed to minimizing American--not Japanese-- losses.[9]

Nuclear operations

The decision on where and when to use the weapons had been delegated to the field commander. It was a requirement, however, that two bombs be delivered in quick succession, so the Japanese could not assume it was a one-of-a-kind weapon. A third weapon would not have been available for several weeks.


If the bombs were not nuclear, but simply of comparable size, weight, shape and specialized construction, preparing to drop them would be no simple task. It was necessary to bring large, highly secret, and extremely costly bombs from the U.S. to the Pacific base. The base needed to have a secure area in which the operation could be staged -- and it was by no means guaranteed that every Japanese soldier had been cleared from the island.

B-29s typically dropped large numbers of relatively small bombs, in the 500 pound range. The aircraft had to be modified to hold large bombs. With the bomb, fuel, and associated equipment, it would be at the very edge of a B-29's takeoff characteristics, so a skilled pilot had to use the longest available runway.

The sudden release of an 8000-10000 pound bomb would cause radical changes in flight characteristics. Even if there were no blast, it would not be difficult to lose control of the aircraft.

The tactical unit

The unit that would actually drop the bombs, the 509th Composite Group, began to organize and plan in September 1944, having its personnel assembled at an isolated base in Westover, Utah in December 1944. Initially, only its commander, Colonel Paul Tibbets, had any idea of the actual mission. Nevertheless, even beyond training, the Group has substantial work to do in developing the tactics of the attack itself. The bombs, especially the Fat Man (nuclear weapon)|Fat Man implosion bomb, were physically large, requiring modifications to the aircraft, and special techniques to lift the bomb into it.

To survive the explosion, the bomber had to make a radical 158-degree turn away from it as soon as the bomb was released. Conventional bombing had no such requirement. As practice for the attacks, crews with no knowledge of the actual nuclear program made attacks on Japan, using a single high-explosive bomb called a "pumpkin", which allowed them to perfect the drop and evasion method in an operational environment. Again, this took time.

Additional members of the tactical Group gradually began to work with Manhattan Project staff on specific issues such as arming and aiming the bombs. Nevertheless, only a handful of airmen had even a slight understanding of the actual mission until just before the Hiroshima strike took off.

A few specialists from the Manhattan Project, such as Captain William "Deke" Parsons, U.S. Navy, would actually fly the mission. Parsons had worked on the detonation system development and would actually arm the bombs in flight. It had been decided that it was too dangerous to try the takeoff with a bomb that might detonate in a crash. This was another decision made only on 5 August. The flight crews carried pistols, but were given cyanide capsules in flight. Having a suicide option could be a defense against torture if captured, but any crewmen with detailed knowledge of bomb principles, such as Parsons, and to a lesser extent the group commanders and the radar bombardiers, simply could not be taken alive.


Given the need for the military to prepare the raids, the effective order was issued by President Truman on 25 July, one day before the Potsdam Declaration.

In orders issued on 25 July and approved by Henry Stimson|Stimson and George C. Marshall|Marshall, Spaatz was ordered to drop the "first special bomb as soon as weather will permit visual bombing after about 3 August 1945 on one of the targets: Hiroshima, Kokura, Niigata and Nagasaki." He was instructed also to deliver a copy of this order personally to MacArthur and Nimitz. Weather was the critical factor because the bomb had to be dropped by visual means, and Spaatz delegated to his chief of staff, Maj. Gen. Curtis LeMay|Curtis E. LeMay, the job of deciding when the weather was right for this most important mission. [10]

Components were still being delivered as the orders worked down the chain of command. The core of the second bomb was delivered to Tinian on the USS Indianapolis (CA-35)|USS Indianapolis (CA-35) on July 26. Four days later, the departing cruiser was sunk by a Japanese submarine, in the worst single-ship disaster in the history of the U.S. Navy. Had she been sunk on the way to Tinian, it might have delayed the attacks by months, if the assumption was that two bombs had to be delivered in quick succession.

The bombings

Four cities, Hiroshima, Kokura, Niigata and Nagasaki, had been left relatively undisturbed, so both the weapon effects could be measured and that they would be psychologically more devastating. Kyoto was taken from the list as a cultural treasure.

So that the aiming and measurements could be most precise, the mission doctrine called for bombing targets that were clearly visible. Individual weather aircraft, therefore, would fly to the targets an hour before the potential raid, and the aircraft with the bomb would be accompanied by one or two observation aircraft. Under this guidance, if a bomb could not be dropped, it would have to be jettisoned in the sea. Nagasaki was bombed by radar rather than lose the bomb, a decision made aboard the bomber and later accepted by higher authority.

A third bomb would not have been available for several weeks. While no target was formally selected for it, and the need was overtaken by the surrender of Japan, a number of historians speculate it would have been aimed at Tokyo, not Niigata or Kokura. There are, however, counterarguments to targeting Tokyo, including leaving a command structure that could surrender, and not enraging the nation by killing the Emperor. It is far more plausible, however, that all further bombs would be reserved for planned tactical nuclear attacks against Japan; there were specific plans for the use of 7 to 9 bombs in Operation Olympic.

Attack on Hiroshima

The Potsdam Declaration had included a warning that must surrender unconditionally or face "prompt and utter destruction." No other more specific warning was given of the nuclear attacks. Hiroshima was bombed on 6 August, and Truman's announcement came afterwards.

The first attack was on the city of Hiroshima. While it was unquestionably a populated city, it also contained dispersed industry, and the headquarters of the Second General Army, which commanded the Japanese defense of Kyushu within the overall Operation KETSU-GO defensive plan. Operation Olympic, the first phase invasion, was targeted at Kyushi.

Actually dropping the bomb was a B-29 named Enola Gay, piloted by Col. Paul Tibbets, who commanded the 509th Composite Group. It was accompanied by two other B-29's taking scientific observations.

The aiming point for the bomb was the Aoki Bridge, selected because it was easily recognized from the air, and near Hiroshima Castle. At 8:15 AM, it detonated 1800 feet/580 meters in the air. The actual ground zero, or surface point under the explosion, was Shima Hospital, which is near the present "A-Bomb Dome" memorial, the former Hiroshima Prefectural Industrial Promotion Hall. The Hall was the only building in the central blast area that was not totally destroyed, and is now a UNESCO World Heritage site. [11]

Estimated explosive yield was 12.5 kilotons, smaller than the Nagasaki bomb that caused fewer casualties. The target areas, however, were quite different: the Hiroshima bomb went off in the center of a flat city, while the Nagasaki weapon exploded over a valley with constraining hills.

Evaluating the effect of a nuclear weapon, based on blast alone, is misleading. A very significant portion of the short-term casualties at Hiroshima were not directly from blast, but from fire. Some of the fires were indirectly caused by blast, as in those that overturned cooking stoves or broke gas lines. The bulk of the firestorm, however, came from the direct thermal effect of the nuclear explosion.[12] It should be noted that major thermal effect is characteristic of airbursts, and should not be applied to plausible nuclear terrorism scenarios.

Attack on Nagasaki

Kokura (city)|Kokura was actually the primary target for the second attack, but weather conditions prevented a bomb delivery there. Nagasaki was the secondary target, and was still largely covered by clouds. The drop was aimed by radar, rather than visual sighting.

The Nagasaki area has more hills than Hiroshima, which channeled the bomb effects. Even though the weapon used was higher in yield (20-22 kilotons) than that used at Hiroshima (12.5 kt), casualties were fewer although the local damage was more intense. Besides the topography, casualties were lower because the bomb was not dropped in the central part of a city, but in a distinctly industrial area. It detonated 1,840 feet above and approximately 500 feet south of the Mitsubishi Steel and Armament Works.

Impact on Japan

The Strategic Bombing Survey did confirm that the weapons had a major psychological effect on the populace:

Prior to the dropping of the atomic bombs, the people of the two cities had fewer misgivings about the war than people in other cities and their morale held up after it better than might have been expected. Twenty-nine percent of the survivors interrogated indicated that after the atomic bomb was dropped they were convinced that victory for Japan was impossible. Twenty-four percent stated that because of the bomb they felt personally unable to carry on with the war. Some 40 percent testified to various degrees of defeatism. A greater number (24 percent) expressed themselves as being impressed with the power and scientific skill which underlay the discovery and production of the atomic bomb than expressed anger at its use (20 percent).[13]

A discussion between Navy Minister Mitsumasu Yonai and Deputy Chief of Staff Sokichi Takagi, two days after Hiroshima, indicates that Yonai, a member of the inner cabinet and peace faction, was more concerned with the threat of a domestic uprising than the impact of the nuclear attack. Yonai mentioned Hiroshima in the context of aggravating the domestic situation, not as a primary event. [14]


  1. Tsuyoshi Hasegawa (2005), Racing the Enemy: Stalin, Truman and the Surrender of Japan, Harvard University Press, ISBN 9780674022416, p. 42-43
  2. Hasegawa, pp. 148-149
  3. 3.0 3.1 Summary Report: (Pacific War), United States Strategic Bombing Survey, 1946
  4. John Ray Skates (1994), The Invasion of Japan: Alternative to the Bomb, University of South Carolina Press, p. 235
  5. , Part V: The Atomic Bomb and American Strategy, The Interim Committee Report, The Manhattan Project: Making the Atomic Bomb, Atomic Archive, funded by National Science Foundation
  6. Science Panel's Report to the Interim Committee: Recommendations on the Immediate Use of Nuclear Weapons, Hiroshima & Nagasaki Remembered, 16 June 1945
  7. A Petition to the President of the United Stats, Hiroshima & Nagasaki Remembered, 17 July 1945
  8. Craven and Cate, 5: 608-14; Thomas R. Searle "'It made a lot of sense to kill skilled workers': The firebombing of Tokyo in March 1945," Journal of Military History 103-134 66, no. 1 (Jan 2002): p. 103-134
  9. Richard Frank, Downfall: The End of the Imperial Japanese Empire (1999)
  10. Louis Morton (1960), Chapter 23: The Decision To Use the Atomic Bomb, in Kent Roberts Greenfield, Command Decisions, Center for Military History, U.S. Army, p. 514
  11. Hiroshima Peace Memorial (Genbaku Dome), UNESCO
  12. Robert C. Harney (September 2009), "Inaccurate Prediction of Nuclear Weapons Effects and Possible Adverse Influences on Nuclear Terrorism Preparedness", Homeland Security Affairs, Naval Postgraduate School Center for Homeland Defense and Security (CHDS) V (3)
  13. United States Strategic Bombing Survey, Summary Report: (Pacific War) (1946) online p. 25
  14. Ward Wilson (Spring 2007), "The Winning Weapon? Rethinking Nuclear Weapons in Light of Hiroshima", International Security 31 (4): 162–179