Although Robert Oppenheimer and his fellow scientists saw the use of the atomic bombs in moral terms, for his president the deployment of the weapons was more transactional. He hoped the weapons would do two things. The first was avoid the enormous casualty bill that beckoned if the main Japanese islands were invaded. The figure bandied around Washington DC at the time was up to one million allied dead and wounded, plus up to ten million Japanese. As an indication of the anticipated casualties, half a million Purple Heart wound medals had been manufactured, whose stockpile is still being gradually eroded to this day. Post-conflict support of any civilian survivors of the mass slaughter was also reportedly beyond allied capabilities for several months.

The second was to get the Imperial Japanese forces to actually surrender, which they very nearly didn’t. Hardliners resisted and it was only the personal decision of Emperor Hirohito to capitulate on 14 August after the release of both atomic weapons, and broadcast the next day, that brought about the end. A last-minute attempted military coup d’état by diehards opposed to such dishonour almost scuppered the surrender and would have continued the war.

After the first and only test of a plutonium device at the Trinity test site on 16 July, there had been much US discussion of a technical demonstration to the Japanese of a live atomic weapon, but this was dismissed as ‘unlikely to bring an end to the war; we see no acceptable alternative to direct military use’. Truman’s risk was that his foes would fight on regardless, or worse, refuse to believe there were any more atomic bombs, and to an extent they were correct. There were plans for further atomic attacks, but another ‘Fat Man,’ based on plutonium and of the kind used over Nagasaki would not be ready, given safe shipment and adequate flying conditions, until late August-September. No further ‘Little Boy’, devices which utilised a Uranium-235 core, but had not been tested before being dropped on Hiroshima, would be ready until December.

The risks were underlined by the fate of the cruiser USS Indianapolis, which had delivered the components for the first ‘Little Boy’ from the United States to Tinian Naval Base, and was torpedoed and sunk on its return leg by the Japanese submarine I-58 on 30 July. Thus, our belief today that a stockpile of atomic weapons alone was certain to bring about the end of the war is not correct. In the event, the arrival of the Soviet Union into the war against Japan on 8 August in the wake of the first bomb and before the second proved as significant in Tokyo’s calculations.

The first phase of the Pacific war had been fought to contain Japan. Only at the Quebec conference of August 1943 was detailed thought given to the subjection of the allies’ Far Eastern adversary, with a rough timetable of an invasion of the Japanese home islands in 1947-48. Soon reassessing that such a prolonged war would be deleterious to morale, a less imprecise schedule of Tokyo’s submission ‘not more than one year after Germany’s surrender’ was then agreed. Meanwhile, US naval policy was to blockade and starve Japan, which by early 1945 was proving remarkably successful. Japanese planners assessed that up to 10 million would have died of starvation by the end of 1946.

In June 1944, allied commanders began detailed invasion planning, under the overall codename of ‘Downfall’. The assault was subdivided into ‘Olympic’, envisaging Okinawa as a staging post to land 700,000 men along 35 beaches on Kyūshū, beginning on 1 October 1945. It was to be followed by ‘Coronet’, a mission commencing on 31 December against the main Japanese island of Honshū, discharging 1.1 million personnel in and around Tokyo Bay. Airbases on Kyūshū would allow land-based air support for Coronet, removing the maritime exposure of costly aircraft carriers. The other main Japanese islands of Hokkaidō in the north and much smaller Shikoku were considered unsuitable for amphibious assaults. The bulk of the invading land forces would be American, but a Commonwealth corps of five divisions landing in the second wave of Coronet would comprise formations not already committed elsewhere, for which British troops in conquered Germany were readied. It would have easily surpassed ‘Overlord’ in Normandy in size, scale and ambition.

Unlike D-Day, when the Germans were genuinely perplexed as to where their opponents would land, the allied plan of leaping from Okinawa to Kyushu, then Honshū, was predictable, and the Japanese defences and dispositions were readied accordingly. Both sides knew this. The Japanese response was Operation Ketsugō (Final Battle), supported by a propaganda campaign calling for ‘the glorious death of one hundred million’. Tokyo archives now reveal that even fortune-tellers were conscripted to predict the expulsion of the allied invasion.

Japan’s remaining four million soldiers and sailors on the main islands, including hardened units withdrawn from Manchuria, were trained to repel any landings, while airmen and sailors with their machines were readied for kamikaze tactics, and civilians formed into home guard militias armed with obsolete firearms and bamboo poles to combat allied land forces. In the 1990s, while touring the Yamagata Dantsu Japanese carpet factory, founded in 1935, but making small-arms ammunition a decade later, I found several of the senior workforce who remembered as 16-year-old schoolchildren being trained in class with bows and arrows, swords, and bamboo lances. ‘I wanted to complete my education’ said one, ‘but was instead given a war club to brain an allied soldier.’ They were expected by both sides to die for their emperor. Thus, Truman was right to see beyond the moralistic arguments, especially as radiation sickness was then unknown, to a simple methodology of ending the Second World War.

There was another important aspect to Operation Downfall, which was the participation of the Soviet Union. At the Yalta conference of February 1945, Stalin was urged to participate by invading the Kuril archipelago and the southern part of Sakhalin island, only 30 miles north of Hokkaido. Although Soviet participation in the invasion of mainland Japan was not expected or even welcomed, the mere threat to Tokyo of invasion by fresh enemy, Russia (even though its navy had little amphibious capability), was hoped to be able to tip the Japanese mind in the direction of capitulation, as later proved the case. The Soviet declaration of war on Japan would eventually occur on 8 August.

With fanatical Japanese resistance and the slow allied movement of troops, ships and resources to the Pacific, the Olympic and Coronet invasion dates eventually slid to 1 December 1945 and 1 April 1946 respectively. Likely allied losses were calculated on the basis of casualties sustained at Okinawa in April-June 1945, where 368 out of 1,300 attacking ships were badly damaged, and another 28 sunk. With Douglas MacArthur as overall land force commander, Chester Nimitz governing maritime assets, and Carl Spaatz overseeing air force activity, it was Operation Coronet that was planned to deliver the final blow to the enemy heartland, ending with the capture of Tokyo. Planning was paused, reevaluated and reapproved on Truman taking over after Roosevelt’s death on 12 April. The new man in the White House presented the finished scheme to his British and Soviet counterparts at the Potsdam conference beginning on 17 July. Stalin (already secretly aware via the Burgess-Philby-Maclean spy ring) approved and Attlee made no alterations on replacing Churchill on 26 July, indeed their lordships at the Admiralty were greatly relieved to find that he meddled and interfered little, compared to the excessive scale of his predecessor.

It was in 2005 that I first met Raymond ‘Hap’ Halloran. I was lecturing on Cunard’s Queen Mary 2 about the last days of the Second World War. Crowded with veterans and their families, it was in mid-Atlantic that I encountered the man who had taken the ship’s most expensive duplex suite. Travelling alone, Hap liked style. He invited me to dinner in the liner’s Japanese restaurant, and with engaging shyness gradually let his guard down.

Only a year younger than my father, after enlisting in the USAAF in 1942 and passing exams as a navigator, Hap had been commissioned as a second lieutenant and joined a crew in collective training on B-29s in Lincoln, Nebraska. He remembered his desk just aft of the plexiglass cockpit, which gave the pilot, co-pilot and bombardier a stunning panoramic view of the outside world. Naming their machine ‘Rover Boys Express’ they flew to Isley Airfield on Saipan, captured in July 1944, and joined the 73rd Bombardment Wing, to begin bombing the Japanese mainland. A talented navigator, at one stage, he flew with Colonel Paul Tibbets, who asked him to join his B-29 group. Faced with leaving the Rover Boys Express, Hap said he’d think it over while he completed a few more missions.

His fourth sortie on 27 January 1945 changed everything. The Rover Boys Express set out for the Musashiho and Nakajima aircraft factories near Tokyo. Fifteen miles west of their target near Mount Fuji, the B-29 was attacked by a twin-engined Kawasaki Ki-45 Toryu (meaning Dragonslayer) fighter. Hap and the rest of the 11-man crew called this type a ‘Nick’. Comfortable in the knowledge that at 27,000 feet, they were flying higher than most Japanese aircraft and air defence fire could reach in their pressurised B-29, the American aviators felt immune from interference and were wearing only shirt sleeves for their long, 15-hour mission.

The Ki-45’s pilot had other ideas. Struggling through the thin air to intercept, he opened fire head on, and unleashed a deadly stream of 37mm cannon shells which smashed the B-29’s plexiglass, took out two engines, all their electronics, intercoms, and depressurised the cabin. Pilot First Lieutenant Edmund G. Smith ordered the crew to bail out and seven including Hap managed to exit successfully. He remembered the remaining four huddled together, praying and opting to stay with the Rover Boys Express than risk a grizzly, prolonged death on the ground at the hands of the Japanese.

At the high altitude his eyelids froze shut and in his shirtsleeves he was conscious only of the cold. With his chute belatedly opening at 3,000 feet, Hap had already made his peace, expecting death. They landed separately, and five including Hap were quickly rounded and severely beaten by civilians armed with sharpened bamboo poles, who would have killed them, but for the intervention of  the Imperial Japanese Army military police troopers. Six crewmates died in the B-29 or were murdered on the ground and never seen again. The survivors were separated and after torture and interrogation, Hap’s captors meted out the ultimate degradation to him of months of solitary confinement in an empty animal cage in Ueno Zoo in central Tokyo.

There seemed so much Hap wanted to unburden himself of, that his tale eventually took several dinners to complete. The next night he wanted to stress his Catholic faith that sustained him in the zoo, while the angry and the curious came to inspect him, their first American, poking him with sticks as they would a wild animal. He lived through the 9-10 March firebombing of Tokyo, which local police reported took the lives of 83,000 (though the final count almost certainly exceeded 100,000), finding it terrifying to be under incendiary bombs released from 279 Superfortresses flown by his colleagues.

This represented a switch to more aggressive bombing tactics by the incoming USAAF commander Curtis Le May, who began a strategy of dropping thousands of M-69 six-pound napalm bomblets from 5-7,000 feet, which consumed many timber-built Japanese conurbations. Previously, most air forces had used thermite or magnesium incendiaries, which were easier to extinguish than burning petroleum jelly. The M-69s were an early cluster munition, with 38 packed into a larger casing, of which a B-29 could carry up to 40.

Hap unravelled these facts of yesteryear to me with ease, the statistics deeply seared into his memory. Surviving what would turn out to be the largest civilian death-toll from a single raid of the war, he tried to convey a sense of the heat and smoke of the firestorm which lasted between 01:00 to 04:00, while he was naked, chained to the cage. Hap was both perpetrator and victim of the allied bomber campaign on Japan, which killed more in a single raid than either atomic bomb. He was behind bars, with only a metal bowl for food, another for water and no washing or toilet facilities. Afterwards, a further 66 conventional raids using incendiaries rained down on Japanese targets, flattening 162 square miles of industrial landscape, killing 250,000 in total.

Over our third dinner together, he told me of his sense of isolation in a world that had surrendered its humanity and knew nothing of the end of the war in Europe, only that, as the Pacific war ground on, his captors grew ever more aggressive and gave him less food. Convinced that he, alongside most of his fellow 285,000 allied military and civilian internees, would be murdered, Hap was eventually transferred to a POW Camp from which he was liberated on 29 August 1945, a few days before MacArthur took the Japanese surrender in Tokyo Bay on 2 September. They were lucky, for orders had already been issued for prisoners to be killed and their guards to afterwards flee. By then, unable to even stand, Hap was unwashed, covered with fleabites, had lost over half his bodyweight and witnessed the majority of his fellow POWs die of malnutrition or ill-treatment. He had to spend a year in a military hospital before regaining enough strength to enter civilian life.

Though pursued by nightmares ever after, eventually he made friends with the Japanese pilot who shot him down, hosted his daughter when she studied in the United States and delivered the eulogy at his funeral in Tokyo, becoming an ambassador for reconciliation between the two nations. Now no longer with us, joining the Great Muster Beyond on 7 June 2011, Hap Halloran, who might have flown with Tibbets on the Hiroshima mission, told me that having faced death so many times, he regarded each day as a ‘bonus day’ from his Maker and his meetings with me, as one who listened and tried to understand, were flatteringly, ‘super bonus days’.

When the Japanese Emperor ordered his forces to surrender in the wake of both atomic bombs and the Russian invasion, it came as a huge shock to the western coalition powers, who had been gearing up for a vast and costly invasion. The civilian and military mindset had fully expected the Japanese to carry on into 1946, while Truman’s decision to use the only two atomic weapons available was based on the merest of remote chances that Tokyo would cave in. In his mind, anything that might help deflect up to a million allied casualties, given all the losses already incurred fighting the Italians, Germans and Japanese, was worth a try, and indeed was a moral decision.

The British Pacific Fleet and Tiger Force of RAF very long-range Lancaster and Lincoln bombers, both created specifically established to support the mostly US invasion of Japan, was quickly wound down. The invasion that never was, Operation Downfall, was immediately cancelled. Airframe Fitter Frank Colenso on Toungoo airfield, in support of 155 Squadron RAF, 140 miles from Rangoon, remembered the moment it all ceased. ‘Near my airfield a battery of long-range guns of the Royal Artillery kept up a deafening bombardment into the hills, only pausing to allow our Spitfires to land, then resuming firing salvo after salvo right up to the 11am ceasefire. Suddenly the incessant noise of the morning’s action stopped. The last aircraft landed, taxied away, parked and cut their engines. The silence was awesome, not a sound to be heard, leaving me with the dawning realisation that, at last, the war was over.’