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Dropping “The Bomb”: Hiroshima and Nagasaki
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In 1942, the United States made the decision to build a weapon unlike anything the world had ever seen. The reason was straightforward and a bit terrifying. Scientists, like Albert Einstein were fleeing Nazi occupied Europe and warned that Germany might be attempting to build an atomic bomb. If Germany succeeded first, the war—and possibly the world—could end in catastrophe. In response, the U.S. government launched a secret program to build an atomic bomb as quickly as possible. That program would become the Manhattan Project, and its result would reshape warfare, global politics, and human history.
The characteristic mushroom cloud of a nuclear explosion.
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Building the Bomb in Total Secrecy
The Manhattan Project was enormous, far larger than most people imagine. The Manhattan Project involved more than 130,000 people spread across secret sites in Tennessee, Washington, and New Mexico. Entire towns were built just to support the work. Most of the people involved had no idea what they were contributing to. Some ran machines, some handled radioactive materials, while others crunched the numbers. Very few ever saw the full picture. All of it was done under strict secrecy and intense pressure during a global war. Even Vice President Harry Truman did not know the Manhattan Project existed until he became president in April 1945.
The Manhattan Project was spread across the country because no single location could handle the entire process. Each site existed for a specific technical reason. In Oak Ridge, Tennessee, massive industrial facilities were built to separate uranium-235 from uranium-238, a task so difficult it required enormous amounts of electricity.
In Hanford, Washington, nuclear reactors produced plutonium, an element that does not occur naturally in usable quantities. Uranium fuel rods were bombarded with neutrons, creating plutonium that then had to be chemically separated in dangerous, highly radioactive conditions.
The scientific heart of the project was Los Alamos, New Mexico, where some of the world’s brightest physicists and engineers designed the bombs themselves. Teams debated calculations, tested explosive shapes, and tried to solve problems that up to that point were just theories. Nearby desert land also provided uninhabited space for the world’s first nuclear tests.
Scientific leadership fell to J. Robert Oppenheimer, a brilliant physicist who had never managed a project of this size. Under immense pressure, he coordinated scientists, engineers, and military officers, many of whom were refugees who had fled fascist Europe and feared what would happen if Germany developed the bomb first.
Manhattan Project Scientists at the 4th anniversary of the development of the Chicago Pile (CP-1), 1946.
US National Park Service
Los Alamos National Laboratory. Established in 1943
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How Atomic Bombs Work
The atomic bomb worked by creating a nuclear chain reaction, and that reaction depended on using unstable atoms and forcing them past critical mass. Atoms fall into two basic types: stable and unstable. In stable atoms, the forces inside the nucleus are balanced. Protons push away from each other, but neutrons hold them together strongly enough that the nucleus stays intact over time.
In unstable atoms, that balance isn't there. These atoms—such as uranium-235 and plutonium-239—have nuclei where the protons push outward more strongly than the neutrons can hold them together. The nucleus is under constant stress. Over very long periods of time—sometimes tens of thousands or even millions of years—unstable atoms can split on their own, releasing small amounts of energy. This slow, natural process is called radioactive decay.
What made the atomic bomb different was that scientists did not wait for unstable atoms to split naturally. They forced large numbers of them to split all at once.
Scientists discovered that when one unstable atom splits, it releases neutrons that can strike nearby unstable atoms and cause them to split as well. If there are only a few atoms, or if they are too spread out, those neutrons escape without hitting anything else and the reaction stops. This is called a subcritical state. No explosion happens.
Reaching an explosion grade critical mass occurs when enough unstable atoms are packed closely together so that, on average, each splitting atom causes at least one more atom to split. At that point, the reaction accelerates. If enough atoms are forced even closer together, the reaction becomes supercritical.
The challenge for the Manhattan Project scientists was attaining the right speed. The fissile material had to be pushed from subcritical to supercritical in microseconds. If it happened too slowly, the reaction would begin early and blow the bomb apart before enough energy was released.
Once the material became supercritical, the chain reaction raced through the atoms in less than a millionth of a second. Trillions of atoms split. The energy that would normally be released slowly over thousands of years was released all at once as heat, pressure, and radiation. That sudden release of energy is what caused the explosion that destroyed entire cities.
Diagram of an atom: Protons and neutrons cluster at the center (the nucleus). In a stable atom the protons are neutrons can keep each other in check.
Credit: How Stuff Works
A small particle called a neutron hits the nucleus of a uranium-235 atom. That impact makes the nucleus unstable, and it splits apart into two smaller nuclei called fission fragments. When the split happens, it releases a large amount of energy, shown by the red zigzag lines, along with more neutrons.
Credit: How Stuff Works
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The Trinity Test
At 5:29 a.m. on July 16, 1945, the first atomic bomb was detonated in the New Mexico desert at a site code-named Trinity. The explosion vaporized the steel tower holding the device and created a fireball that rose more than seven miles into the sky. And yes, we literally mean that the heat didn’t just melt the metal, it turned it into gas. Observers miles away felt the heat and watched the desert light up brighter than sunrise.
Sand beneath the blast melted into green radioactive glass later called trinitite. Windows shattered over a hundred miles away. Some scientists celebrated. Others stood in stunned silence. The weapon worked, and there was no longer any doubt about what it could do.
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The Decision to Drop "the Bomb"
Germany surrendered in May 1945 but Japan did not. By that point, the Pacific War had already shown what the final stage of the conflict would look like. At Iwo Jima, nearly 7,000 Americans were killed fighting for a single island. Almost the entire Japanese garrison died. At Okinawa, more than 12,000 Americans were killed. Roughly 100,000 Japanese soldiers died, along with tens of thousands of civilians. Japanese forces rarely surrendered. Japanese soldiers had been trained that death in battle was more honorable than surrender, even when the odds of winning were impossible.
American bombers were already devastating Japan’s cities. In March 1945, firebombing raids on Tokyo killed an estimated 100,000 people in a single night. By summer, dozens of cities had been partially destroyed. U.S. planners expected the bombing to continue city by city until Japan was forced to stop fighting.
U.S. military planners warned that invading the Japanese mainland would be far worse. Intelligence reports and battlefield experience suggested that Japanese forces would fight to the death. Estimates projected hundreds of thousands of Allied casualties and millions of Japanese deaths. Civilians were being organized to support the defense of the homeland, trained to resist invasion with improvised weapons like bamboo spears, farm tools, and even home-made grenades.
Military officials drew up a list of potential atomic targets, including Hiroshima, Kokura, Niigata, and Nagasaki. These cities had not yet been heavily bombed and would clearly show the power of an atomic bomb. The plan was clear: if Japan did not surrender after one bomb, more cities would be destroyed. Additional atomic bombs were already being prepared for use later in August 1945.
President Harry S. Truman, who had taken office after Franklin Roosevelt’s death in April, faced a narrow set of choices. Continued firebombing, naval blockade, full invasion, or atomic attacks. Truman approved the use of the atomic bomb because U.S. leaders believed it offered the fastest way to force surrender in a war where "fight to the death" was not just a catchy slogan.
This map shows the cities bombed and percentage destroyed using conventional and firebombs.
Credit: National World War II Museum in New Orleans.
This is NOT the aftermath of an atomic bomb. This photo taken in March,1945 shows the devastating effects of repeated fire bombings on Tokyo.
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Hiroshima: August 6, 1945
At 8:15 a.m. on August 6, 1945, Hiroshima was waking up to what seemed like a typical summer morning. Streetcars were running. Shops were opening. Children were already seated in classrooms. The city had not experienced heavy bombing, and many residents believed it had been spared. High above the city, a B-29 bomber called the Enola Gay was approaching its target after hours of flight from Tinian Island. Inside its bomb bay was a uranium-based atomic weapon known as Little Boy. Most people below had no warning and no reason to expect that their city was about to become the first target of a nuclear weapon.
When the bomb detonated above the city, survivors described a blinding white flash followed instantly by a crushing pressure wave. Buildings collapsed. People were thrown across rooms. Temperatures near the blast center reached several thousand degrees Fahrenheit. Those closest to the explosion were killed immediately, incinerated or buried under debris. In some places, dark silhouettes were burned into stone where bodies had briefly blocked the heat.
Fires ignited across the city within minutes. Wooden homes and paper walls fed a massive firestorm. Survivors wandered through the ruins with severe burns, holding their arms away from their bodies because burned skin peeled when touched. Thousands fled toward rivers to cool their burns or escape the flames. Many drowned or collapsed along the banks.
Medical care collapsed almost instantly. Hospitals were destroyed. Doctors and nurses were dead or injured. Survivors crowded into schools, temples, and open fields with no supplies and no understanding of radiation. People who appeared unharmed began vomiting hours later. Hair fell out. Gums bled. Internal organs failed.
By the end of 1945, an estimated 140,000 people—nearly half the city’s population—were dead.
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Nagasaki: August 9, 1945
Three days after Hiroshima, Japan still had not surrendered. On the morning of August 9, 1945, the United States prepared to use a second atomic weapon. This bomb, a plutonium-based device called Fat Man, used a more complex implosion design that compressed plutonium inward from all sides to trigger nuclear fission. The bomber’s primary target was the industrial city of Kokura, but heavy cloud cover prevented visual confirmation. After several failed passes and with fuel running low, the crew diverted to the secondary target, Nagasaki, where the bomb was dropped.
Again, tens of thousands were killed instantly. Buildings collapsed or were vaporized. Steel frames bent. Fires spread through the valley. Survivors described choking smoke, confusion, and people wandering the streets crying out for family members.
Again, medical facilities were overwhelmed. Doctors treated burns and performed amputations without anesthesia or clean water. Radiation sickness followed the same pattern as Hiroshima, appearing days and weeks later. By the end of 1945, roughly 70,000 people in Nagasaki were dead.
That same day, the Soviet Union declared war on Japan and invaded Japanese-held territory in Manchuria.
Taken 15 minutes after the atomic bomb detonation over Nagasaki from Koyagi-jima Island. The earliest photograph taken from the ground.
Credit: Hiromichi Matsuda
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Japan Surrenders
In the days after Hiroshima and Nagasaki, Japan’s leadership was deeply divided. Some military officials wanted to continue fighting, arguing that surrender would dishonor the nation. Others believed the situation was now hopeless. The atomic bombings had destroyed two cities in seconds, and on August 9 the Soviet Union declared war on Japan and invaded Japanese-held territory in Manchuria. Japan now faced the possibility of continued nuclear attacks, a Soviet invasion from the north, and an American invasion from the south.
The final decision came from the emperor himself. On August 15, 1945, Emperor Hirohito addressed the Japanese people by radio for the first time in history. Most citizens had never heard his voice. He announced that Japan would accept the Allied terms of unconditional surrender. He cited the overwhelming destructive power of a new weapon and warned that continuing the war would only lead to the complete destruction of the nation and the suffering of its people.
The broadcast stunned the country. Soldiers were ordered to lay down their arms. Fighting stopped across the Pacific. For millions, the war ended not with celebration, but with shock, relief, and uncertainty about what would come next.
When news of Japan’s surrender reached the United States, Americans went wild with joy and relief. Church bells rang, strangers hugged one another, crowds filled streets in cities across the country. In New York, Times Square was packed as thousands of people rushed into the streets to celebrate. After nearly four years of intense fighting, the war was finally over.
For many Americans, the celebration was about relief more than triumph. Families understood that more soldiers would no longer be called into combat. An invasion of Japan was no longer coming. Servicemen overseas celebrated knowing they would finally be going home.
The celebrations did not erase the cost of the war. Over 400,000 Americans had died. But on that day, all that mattered was that the fighting had ended. The United States had won the war, and a long chapter of loss, rationing, and fear was finally over.
This famous photograph shows a US sailor pulling a total stranger into a kiss. This spontaneous moment occurred during V-J Day celebrations in Times Square on August 14, 1945, when news spread that Japan had surrendered.
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Long-Term Consequences of the Atomic Bomb
By the end of 1945, an estimated 200,000 people were dead as a result of the atomic bombings—about 140,000 in Hiroshima and 70,000 in Nagasaki. Some had been killed instantly. Many others survived the initial explosion but died weeks or months later from severe burns, injuries, and radiation sickness. People who seemed fine at first often grew weaker and their hair began to fall out.
The effects of radiation poisoning lingered long after the fires burned out. In the years after the bombings, survivors were more likely to develop leukemia, with cases rising about two years later and peaking in the early 1950s. Other types of cancers followed. Thyroid, breast, lung, and stomach cancers appeared at higher rates for decades. The closer someone had been to the blast, the higher their risk. Children and the elderly were especially vulnerable.
Survivors, known as hibakusha, often lived with chronic illness, extreme fatigue, weakened immune systems, and long-term psychological trauma. Many hibakusha also faced discrimination because so little was understood about radiation. Survivors were often treated as social outcasts. Some were denied jobs. Others had engagements broken off or were rejected for marriage because of fears that radiation sickness might be contagious or that exposure could be passed on to future children.
Fears of birth defects were widespread after the bombings, but long-term studies did not find evidence of widespread inherited genetic damage passed on to survivors’ children. Some pregnant women who were directly exposed to radiation—especially early in pregnancy—did give birth to children with developmental delays or physical disabilities caused by direct exposure to radiation in the womb, not genetic changes that were inherited.
But sadly, even 80 years later, some Hibakusha continue to hide their past, especially in rural areas or from those of the older generation out of fear of being stigmatized.
The 80th anniversary of the devastating attack on Hiroshima was commemorated with survivors recalling their experiences. NBC News' Janis Mackey Frayer reports from Hiroshima, detailing the experiences remembered by those who lived it.
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Why It Matters
Historians still argue about whether the atomic bomb was necessary. Some believe it prevented an invasion of Japan and saved lives. Others argue Japan was already close to surrender or that other options should have been tried first. That debate continues. What is not debated is the impact. The atomic bomb permanently changed how war works.
After 1945, war between major powers became something leaders feared starting at all. During the Cold War, the United States and the Soviet Union built massive nuclear arsenals. The goal was not to use them, but to stop the other side from attacking. This idea—deterrence—meant that a full-scale war could quickly turn into nuclear destruction. Events like the Cuban Missile Crisis showed how close misjudgment or panic could bring the world to disaster.
That fear did not disappear when the Cold War ended. Today, concern has shifted to smaller states or extremist groups gaining access to nuclear weapons. Countries like North Korea continue to pursue nuclear capabilities despite international pressure. Modern military planning now has to consider not just winning wars, but avoiding actions that could trigger a nuclear one. The atomic bomb still shapes global decisions because its use would not just end a battle—it could end everything with the push of a button.
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