Inside US President Harry Truman’s decision to bomb Hiroshima and Nagasaki

As historian Carl Haskins warned in Foreign Affairs in 1946, “the bomb provides the greatest temptation to aggression ever offered an ambitious people or an unscrupulous leader… the traditional modes of national defence are clearly inadequate.”

Although nuclear weapons have thus served as a deterrent, the decision to unleash such a force of devastation was difficult at the time. Ultimately, the United States decided to drop two nuclear bombs on Hiroshima and Nagasaki, the first and last of their kind ever to be used on a sovereign state.

Truman later declared, “the final decision of where and when to use the atomic bomb was up to me. Let there be no mistake about it. I regarded the bomb as a military weapon and never had any doubt that it should be used.”

Yet the choice was fraught, shaped by a brutal war, a stubborn enemy, fears of Soviet ambition, and dissenting voices.

By mid-1945, Japan was battered but unbowed. A US naval blockade had crippled its economy, and B-29 firebombing raids had killed roughly 3,33,000 civilians, levelling cities. Yet Japan’s military rejected the Potsdam Declaration’s call for unconditional surrender in July 1945. At that point, a full scaled invasion seemed like the only alternative.

However, Kamikaze attacks and fierce resistance at Iwo Jima (6,200 US deaths) and Okinawa (13,000 US deaths) foreshadowed a brutal conflict. Military planners estimated that the planned assault on Japan’s home islands could cost 250,000 to over 1 million American lives. Japan’s losses, civilian and military, would likely be even more devastating.

Truman, haunted by these projections, wrote, “my object is to save as many American lives as possible.”

Not everyone agreed.

Future President Dwight Eisenhower, then Supreme Allied Commander in Europe, said that he opposed the bomb for moral reasons and believed that Japan was already close to surrender.

There were also domestic political pressures at play. The Manhattan Project had cost nearly two billion dollars. “It was my reaction,” wrote Truman’s Chief of Staff Admiral William Leahy, “that the scientists and others wanted to make this test because of the vast sums that had been spent on the project. Truman knew that, and so did other people involved.”

Additionally, Leahy called the bombings “barbarous,” arguing that Japan was already defeated and factions of its government could be coaxed into a peaceful surrender. He believed the by using the bomb, the US adopted “an ethical standard common to the barbarians of the Dark Ages.”

Scientists at the Chicago Metallurgical Laboratory, tasked with building the bomb, were also concerned. In June, 64 scientists from the lab petitioned Truman to demonstrate the bomb on a barren island first. “It was almost as if they hoped the bomb would not work after it was completed,” recalled physicist John A Simpson.

But the Interim Committee advising Truman, rejected this, fearing failure. Stimson agreed: “Nothing would have been more damaging… than a warning or demonstration followed by a dud.”

For J Robert Oppenheimer, scientific director of the Manhattan Project, there was little question the bomb would be used. “We always assumed if they were needed, they would be used,” he later said. And the strategic rationale extended far beyond military necessity.

Geopolitically, the bomb offered a way to end the war without Soviet help. At Yalta, Soviet Premier Joseph Stalin pledged to attack Japan by August 15. But the July 16 Alamogordo test confirmed the bomb’s power, and leaders saw a chance to avoid Soviet occupation of Japan, as they had in Germany.

As historian Rufus Miles later observed, “American officials realised that they had the means to end the war very quickly… before the Soviets could effectively state a claim for the joint occupation of Japan.”

The Interim Committee, on June 1, recommended using the bomb without warning on a military target surrounded by civilians to deliver “a tremendous shock,” as Stimson put it, to “extract a genuine surrender from the Emperor and his military advisers.”

At 2:45 am, on August 6th 1945, the B-29 Enola Gay dropped a uranium bomb, “Little Boy,” on Hiroshima. Forty-five seconds later, the city was erased, killing 80,000 instantly. When news of Hiroshima’s destruction reached Truman aboard the cruiser Augusta, he issued a cool response. “If they do not now accept our terms, they may expect a rain of ruin from the air, the like of which has never been seen on this earth.” Yet Japan refused to surrender.

As Stalin promised, the Soviet Union declared war on Japan on August 8. The next day, a second bomb devastated Nagasaki.

Hirohito, citing “a new and most cruel bomb” in his surrender broadcast, warned that continued fighting risked “the total extinction of human civilization.”

Public support was overwhelming, with a 1945 Gallup poll showing 85 per cent of Americans approved the bombings. Yet privately, Truman’s resolve wavered. His grandson Daniel later claimed Truman admitted regret, saying, “Hell, yes,” when asked if he felt remorse.

As Robert McNamara, who would later serve as Secretary of Defence, explained in a 2003 documentary, even those who carried out the bombings understood the moral line they had crossed.

McNamara had served under General Curtis LeMay, who oversaw the firebombing of Japanese cities and authorised the use of the atomic bombs. LeMay told him, “if we’d lost the war, we’d all have been tried as war criminals.” McNamara agreed. “He, and I’d say I, were behaving as war criminals.”

The United States Strategic Bombing Survey, completed after the war, concluded that Japan likely would have surrendered before the end of 1945, even without the atomic bomb or an invasion. But at the time, such forecasts were uncertain. The fear of prolonged war, massive casualties, and Soviet encroachment dominated American thinking.

Moreover, as Truman would later write to Samuel McCrea Cavert, Japan’s attack on Pearl Harbour and treatment of prisoners of war justified extreme action. “When you have to deal with a beast,” Truman wrote, “you have to treat him as a beast.”