Hiroshima & Nagasaki: The Atomic Bombs That Ended WWII
On two August mornings in 1945, American planes dropped atomic bombs on two Japanese cities. The world has never been the same. But the story most people carry around — a war ended, lives saved, a clean moral equation — is a much thinner version of what actually happened. This is the fuller account: the science, the politics, the targets, the people on the ground, and the argument that has never really stopped.
By BookOfWorldHistory·June 2, 2026·History·15 min read · 2,830 words
Originally published at: https://www.bookofworldhistory.com/blog/hiroshima-nagasaki-atomic-bombs-ended-wwii
On two August mornings in 1945, American planes dropped atomic bombs on two Japanese cities. The world has never been the same. But the story most people carry around — a war ended, lives saved, a clean moral equation — is a much thinner version of what actually happened. This is the fuller account: the science, the politics, the targets, the people on the ground, and the argument that has never really stopped.
Two dates. Two cities. A combined death toll somewhere between 150,000 and 246,000 people, the majority of them civilians going about an ordinary morning.
The atomic bombings of Hiroshima and Nagasaki are among the most written-about events of the twentieth century, and also among the most contested. The basic facts are agreed upon. Everything around them — why the bombs were dropped, whether they needed to be, what they actually accomplished, what they set in motion — has been argued over ever since, by historians, by governments, by the survivors themselves.
What follows is the history laid out as clearly as the record allows: where the bombs came from, who decided to use them and why, what happened in those two cities when they fell, and why the argument still has not been settled nearly eighty years later.
The cloud that rose over Hiroshima on the morning of August 6, 1945, was visible for miles. Crew members aboard the accompanying observation aircraft described what they saw as something none of them had words for. Below it, a city of 340,000 people had been transformed in an instant.
How the Bomb Got Built
The possibility of an atomic weapon became real in 1938, when German physicists Otto Hahn and Fritz Strassmann demonstrated nuclear fission — the splitting of a uranium atom that releases enormous amounts of energy. The scientific community understood immediately what this could mean if someone turned it into a weapon.
The fear driving the American program from the start was Germany. Many of the scientists who eventually worked on the Manhattan Project were Jewish refugees from Nazi Europe — people who had fled precisely the regime they now worried might be developing such a device. In 1939, Albert Einstein and physicist Leo Szilard wrote directly to President Roosevelt warning him of the danger and urging the United States to begin its own research before Germany got there first.
Work was slow at first. The real acceleration came in late 1941 when a British scientific committee reported that a workable bomb would require far less uranium-235 than previous estimates had suggested — measured in kilograms, not tons. That changed the calculation. Roosevelt transferred the program to the U.S. Army Corps of Engineers, and what became known as the Manhattan Project began in earnest.
At its peak the project employed over 125,000 people across dozens of sites, at a cost of more than two billion dollars. Most workers had no idea what they were working on. The scientific design work was concentrated at Los Alamos, New Mexico, under physicist J. Robert Oppenheimer. Two separate bomb designs were developed in parallel: Little Boy, a uranium-235 gun-type device, and Fat Man, a more complicated plutonium implosion weapon. The first test of any nuclear device — a Fat Man design, detonated in the New Mexico desert on July 16, 1945 — exceeded everyone's expectations. Germany had already surrendered by then. The bomb, if it was going to be used, would be used against Japan.
The State of the Pacific War in 1945
By the summer of 1945 the outcome of the Pacific War was not in question — Japan was going to lose. What remained very much in question was how long it would take and at what cost.
The battles of 1945 had already been brutal beyond earlier expectations. At Iwo Jima, nearly 99 percent of the 21,000 Japanese defenders died rather than surrender. At Okinawa, fought between April and June, 94 percent of the Japanese and Okinawan troops defending the island were killed; the fighting also killed roughly 100,000 Okinawan civilians. American battle casualty rates were climbing, not falling, as the fighting moved closer to the Japanese home islands.
American military planners had already drawn up Operation Downfall, the plan for the actual invasion of Japan, scheduled to begin in October 1945 with landings on Kyushu. Casualty estimates for the operation varied widely depending on who was doing the estimating and what assumptions they used, but all of them were high — hundreds of thousands of American dead and wounded, and Japanese military and civilian deaths that some estimates put in the millions. Japan had 2.3 million troops prepared for home island defense, backed by a civilian militia of 28 million. The Japanese military was training civilians — women, the elderly, children — to fight with sharpened bamboo sticks.
At the same time, the conventional bombing campaign against Japan was already devastating. General Curtis LeMay's firebombing raids had burned out large sections of 64 Japanese cities. The single deadliest bombing raid of the war — a firebombing of Tokyo on March 9–10, 1945 — killed an estimated 100,000 people in one night and destroyed 16 square miles of the city. By mid-1945, Japan had essentially no ability to defend its own airspace; the Japanese military had stopped scrambling fighters to intercept American bombers to conserve fuel for the expected invasion.
By the summer of 1945, American B-29s operated over Japan with almost no opposition. The conventional firebombing campaign had already devastated 64 cities — yet Japan's military leadership showed no sign of unconditional surrender.
Choosing the Targets
Selecting which cities to bomb was itself a deliberate process. A Target Committee was formed that drew up criteria: the city had to be large enough that the damage could be properly assessed; it had to have significant military or industrial value; and — critically — it could not already have been heavily damaged by conventional bombing, because the whole point was to measure what the new weapon could do on its own.
Five cities were nominated initially: Kokura, Hiroshima, Yokohama, Niigata, and Kyoto. Kyoto was the most controversial choice. Secretary of War Henry Stimson personally intervened to remove it, arguing that destroying Japan's ancient cultural and religious capital would poison the peace that would eventually have to follow. He brought the matter to President Truman directly. Kyoto came off the list. Nagasaki was added in its place on July 25.
Hiroshima was, by the standards the committee had set, a good target. It was a significant military logistics and embarkation center — the headquarters of Field Marshal Hata's Second General Army, responsible for defending all of southern Japan, was located there. The city had also been largely untouched by the conventional bombing campaign, which meant the atomic bomb's effects could be cleanly observed. The surrounding hills were noted in the targeting documents as likely to produce a focusing effect, concentrating the blast.
Significantly, the target cities were deliberately kept off the conventional bombing list in the weeks before the atomic missions, so that the destruction would be attributable solely to the new weapon.
August 6: Hiroshima
The B-29 that carried the Little Boy bomb was named Enola Gay, after the mother of its pilot, Colonel Paul Tibbets. It lifted off from the island of Tinian in the Mariana Islands at 2:45 in the morning on August 6, 1945, accompanied by two other B-29s — one carrying scientific instruments, one for photography.
Hiroshima that morning was a city going about its business. An air raid alert had been sounded earlier, then lifted. When the Enola Gay appeared over the city at just past eight in the morning, Japanese radar operators saw only a small number of aircraft — consistent with a reconnaissance mission rather than an attack. No alarm was raised.
At 8:15 a.m. local time, bombardier Thomas Ferebee released Little Boy. The bomb fell for 44 seconds before detonating at approximately 580 meters above the city, almost directly over a surgical clinic near the center of town. The yield was equivalent to roughly 16,000 tons of TNT.
The accounts from survivors describe a flash of light — pika, in Japanese — followed by a massive pressure wave. For those near enough to the hypocenter, there was no experience of these things sequentially; they were simply destroyed. The radius of total destruction extended about a mile in every direction. Fires ignited across the city and merged into a firestorm.
Over 90 percent of Hiroshima's doctors and 93 percent of its nurses were killed or injured in the blast — most had been working in the downtown area closest to the explosion. The one doctor who remained functional at the city's Red Cross Hospital treated patients through the day with almost no supplies. Survivors with severe burns walked through the ruins looking for help, many not yet aware that the entire city had been hit simultaneously. A black rain fell an hour or two after the explosion — a tarry mixture of ash, radioactive fallout, and water.
Estimated deaths in Hiroshima by the end of 1945: between 90,000 and 166,000 people.
Hiroshima after the bombing. The city's reinforced concrete buildings at the center of the blast remained structurally standing — the bomb detonated in the air, directing force downward — while the surrounding neighborhoods of timber-framed houses were obliterated or burned.
Three Days Later: Nagasaki
Japan's war council met after Hiroshima and learned from its own physicists that the city had been destroyed by a nuclear weapon. The military leadership's response was to endure. Admiral Toyoda, Chief of the Naval General Staff, estimated that the Americans probably had only one or two more such bombs — better to absorb them than to surrender unconditionally.
The second mission, on August 9, went badly from the start. The B-29 Bockscar, piloted by Major Charles Sweeney, took off for Kokura — the primary target — but found the city covered by clouds and smoke drifting from a previous bombing raid on a nearby city. Three passes over Kokura produced nothing the bombardier could visually confirm. Anti-aircraft fire was intensifying. Fuel was running low because of a pump failure that had left 640 gallons unusable in a reserve tank. Sweeney turned for the secondary target: Nagasaki.
Nagasaki was also partially clouded. Sweeney was close to ordering a radar drop — less accurate, and against operational orders — when a break in the clouds at the last moment allowed bombardier Kermit Beahan to sight the target visually. The Fat Man bomb, containing about five kilograms of plutonium, detonated at 11:02 a.m. at roughly 500 meters above the city's industrial valley.
The hills surrounding Nagasaki's Urakami Valley actually contained the blast to some degree — areas of the city outside the valley were partially shielded. Even so, the destruction within the valley was near-total. Of 7,500 workers inside the Mitsubishi munitions plant, 6,200 were killed. The Roman Catholic Urakami Cathedral, the largest cathedral in Asia at the time, was completely destroyed. A community of Catholics who had maintained their faith secretly for over 200 years during a period of Japanese religious prohibition was largely wiped out in a single morning.
Estimated deaths in Nagasaki by the end of 1945: between 60,000 and 80,000 people.
After the mission, Bockscar landed at Okinawa on fumes — one engine died on final approach, another died before the aircraft rolled to a stop. Nobody was there to greet them.
The Urakami Valley in Nagasaki after the bombing. The hills surrounding the valley contained the blast to some degree — but within the valley itself, the destruction of the industrial district and surrounding neighborhoods was almost complete.
Japan's Surrender and What Actually Caused It
Japan announced its intention to surrender on August 15, 1945 — six days after Nagasaki, and in the same window as the Soviet Union's declaration of war against Japan and the launch of a massive Soviet invasion of Japanese-occupied Manchuria.
That timing matters for one of the central historical arguments. For decades the dominant American account held that the atomic bombs caused Japan's surrender by shocking its leadership into capitulation. Historian Tsuyoshi Hasegawa and others have since argued that the Soviet entry into the war played an equally or more decisive role — because Japan's strategy for ending the war on acceptable terms had depended on using the Soviet Union as a neutral mediator. Once the Soviets invaded, that path closed entirely.
Emperor Hirohito's recorded surrender announcement on August 14 specifically cited the enemy's use of a new and cruel bomb as a reason for ending the war — but his private communications to the imperial family on August 12, and his recorded Rescript to Soldiers and Sailors delivered three days after the surrender announcement, told a different story: in that second message, it was the Soviet entry that he described as endangering the foundation of the empire, with no mention of atomic bombs.
What historians can say with confidence is that Japan's leaders were weighing multiple catastrophic pressures simultaneously — the atomic bombs, the Soviet invasion, the continued conventional bombing, and a naval blockade that was slowly strangling the country's food supply. Which of these was decisive, and in what proportion, remains genuinely disputed.
The Hibakusha — Those Who Survived
The Japanese government has officially recognized approximately 650,000 people as hibakusha — the Japanese word meaning, roughly, explosion-affected people. As of early 2025, around 99,000 were still alive, most of them in their eighties and nineties.
For many survivors, the bombing was only the beginning of the ordeal. Those who appeared uninjured in the first days sometimes died within weeks from acute radiation syndrome — a condition the doctors of 1945 had no framework to understand or treat. People bled from their gums and lost their hair. The medical establishment, including a New York Times reporter who initially covered it, was pressured to dismiss early reports of radiation sickness as Japanese propaganda.
Survivors who lived faced decades of social discrimination in Japan. There was widespread and unfounded public belief that the effects of the bombs were hereditary or even contagious — fears that made hibakusha difficult to employ and, in many cases, impossible to marry. Research conducted over subsequent decades by the Atomic Bomb Casualty Commission and its successor organization found no statistically significant increase in birth defects among children born to survivors after the war. But the stigma persisted regardless.
Long-term health studies found elevated cancer rates — particularly leukemia — among those who had been closest to the hypocenters. Among the roughly 80,000 survivors monitored in a major longitudinal study, researchers identified a statistically significant excess of several hundred cancer deaths attributable to radiation exposure over the following decades. The average reduction in lifespan, according to a 2016 meta-analysis, was measured in months rather than years for the broader survivor population — though for the most heavily exposed, the consequences were far more severe.
The Debate That Has Never Really Ended
The ethical argument over the bombings has several distinct threads that often get tangled together.
The most common American defense is that the bombs prevented the invasion of Japan, which would have cost far more lives on both sides than the bombings themselves. The casualty estimates for Operation Downfall were genuinely terrifying, and the ferocity of Japanese resistance at Iwo Jima and Okinawa gave no reason to think the home islands would be different. Secretary of War Stimson later wrote of saving a million casualties. Whether his math was right or wrong, the political and military context makes clear that American leadership genuinely believed the alternative to the bomb was a land invasion of appalling cost.
The counterarguments take several forms. Some historians argue Japan was already close to surrender through a combination of the naval blockade and conventional bombing, making the atomic bombs unnecessary. Others point out that a demonstration bombing — over an uninhabited area, or a military installation — might have produced surrender without the mass civilian deaths. The records of the Interim Committee that decided against a demonstration survive; the reasoning was that the Japanese might move Allied prisoners to the site, that a failed demonstration would be worse than no demonstration, and that shock value was considered essential.
A separate line of criticism focuses on the deliberate targeting of cities — civilian centers — rather than purely military objectives, and whether this constitutes a war crime under the laws of war that existed at the time. The 1963 Tokyo District Court ruling found the bombings illegal under international law as indiscriminate bombardment of undefended cities, though it denied survivors compensation on separate grounds.
The Cold War dimension adds another layer. Historian Gar Alperovitz argued in 1965 that the bombings were partly intended to intimidate the Soviet Union and establish American dominance at the start of the postwar world. The evidence for this as a primary motive is debated; the evidence that American policymakers were aware of the geopolitical message the bombs would send is not.
What the Bombs Started
The American monopoly on nuclear weapons lasted four years. In September 1949, the Soviet Union detonated its first atomic bomb. The nuclear arms race that followed shaped the next four decades of world history more completely than almost any other single factor.
By 1986, the United States had 23,317 nuclear warheads and the Soviet Union had 40,159. As of 2019, the two countries still controlled more than 90 percent of the world's estimated 13,865 nuclear weapons. Nine nations now have nuclear capability. The bombs that destroyed Hiroshima and Nagasaki — devices that shocked the world with their destructive power — are today considered small tactical weapons compared to what currently exists in various national arsenals.
Hiroshima and Nagasaki were rebuilt. Both cities have grown well past their prewar populations. The Hiroshima Peace Memorial — the bombed-out Genbaku Dome, preserved as it stood after the blast — was designated a UNESCO World Heritage Site in 1996. The Nagasaki Atomic Bomb Museum opened in the 1990s. Each year on August 6 and August 9, ceremonies are held in both cities. The memorials in each city maintain running lists of identified hibakusha dead — names added annually as survivors pass away. As of August 2025, the combined lists record more than 550,000 names.
The argument over what those two mornings in 1945 meant — whether they were necessary, whether they were justifiable, whether they were a warning or an invitation — has outlasted almost everyone who was there. It is unlikely to be settled. What can be said with certainty is that the world the bombs created is still the one we're living in.
The Genbaku Dome — the ruins of the Hiroshima Prefectural Industrial Promotion Hall, 150 meters from the bomb's hypocenter — was preserved deliberately as it stood after the blast. It was designated a UNESCO World Heritage Site in 1996 and remains the most visited memorial to the atomic bombing.