12 Facts About the Atomic Bombings of Hiroshima and Nagasaki

In early August 1945, the American military dropped two atomic bombs dubbed “Little Boy” and “Fat Man” on the Japanese cities of Hiroshima and Nagasaki, becoming the only use of nuclear weapons in history (thus far). 

The bombs had been developed by the clandestine Manhattan Project, spearheaded by physicist J. Robert Oppenheimer, between 1942 and the summer of 1945. President Harry Truman ordered the bombs dropped on Japan in an effort to end the carnage of World War II. Though Truman’s decision was initially well received by the war-weary American public, his choice has become a topic of widespread contention as to whether the bombings were necessary or justified. 

Eighty years after one of the most destructive acts in world history, here are the essential facts to know.

1. Development of the atomic bomb began with a letter to President Franklin Roosevelt.
2. The decision to deploy atomic weapons rested with Harry Truman.
3. Some administration officials wanted to bargain with the bomb.
4. Leo Szilard and other Manhattan Project scientists tried to dissuade Truman from using the atomic bombs on Japan.
5. The Allies warned the Japanese people of impending destruction.
6. The atomic bombs dropped on Hiroshima and Nagasaki were slightly different.
7. Nagasaki wasn’t the original second target.
8. Most sources claim between 110,000 and 210,000 people were killed in Hiroshima and Nagasaki.
9. Survivors faced hardship and discrimination.
10. Approximately 10 to 20 percent of the victims were Korean forced laborers.
11. The bombings weren’t the only reason Japan surrendered to the Allies.
12. Einstein deeply regretted his role in kickstarting the Atomic Age.
13. Frequently Asked Questions
14. What were the dates of the Hiroshima and Nagasaki bombings?
15. What was the death toll in Hiroshima and Nagasaki?
16. Did the Hiroshima and Nagasaki bombings end the war?

Development of the atomic bomb began with a letter to President Franklin Roosevelt.

The United States’ efforts to develop atomic weapons began in 1939, when physicists Leo Szilard and Albert Einstein co-signed a letter to President Franklin D. Roosevelt warning him of Germany’s potential to develop nuclear weapons of its own. (The Nazis were trying to develop nuclear weapons, but they never succeeded.) By October 1941, Roosevelt had approved military funding for accelerated research into atomic research and the seed of the Manhattan Project was sowed. 

The decision to deploy atomic weapons rested with Harry Truman.

FDR died from a cerebral hemorrhage on April 12, 1945, and Vice President Harry Truman was sworn in as president. He was then told about the top-secret Manhattan Project and approved its continuation. 

When the Trinity Test, the first successful detonation of an atomic bomb, took place in New Mexico on July 16, 1945, Truman was at the Potsdam Conference with leaders of the Allied forces to discuss the demilitarization of Nazi Germany. (Germany had surrendered unconditionally to the Allies on May 8.) Truman officially authorized the use of the atomic bombs on Japan on July 25. The following day, Truman, President Chiang Kai-shek of China, and UK Prime Minister Winston Churchill issued the Potsdam Declaration, a statement demanding Japan’s unconditional and immediate surrender.

Some administration officials wanted to bargain with the bomb.

Japan did not surrender, but its aims were not clear [PDF], and Truman considered what lay ahead: a potential Allied invasion of Japan and the loss of even more American servicemembers’ lives. He decided that deploying the atomic bombs was the best path forward to minimize U.S. military losses while forcing an unconditional Japanese surrender. Though many in his administration agreed, some officials were concerned that it would reveal their hand, and instead encouraged using the bomb as a bargaining chip to intimidate Japan into surrendering peacefully. At the same time, Truman and his administration also believed dropping the bomb would demonstrate American military power to the Soviet Union amid the growing Cold War. 

Leo Szilard and other Manhattan Project scientists tried to dissuade Truman from using the atomic bombs on Japan.

In the early days of the Manhattan Project, much of the initiative’s research was motivated by beating the Nazis’ efforts to develop similar technology. After Germany surrendered to the Allies in May 1945, which revealed the Nazis had no atomic weapons, Leo Szilard—who had once advocated for such research—initiated a petition urging Truman not to use the bomb on Japan (at least not without proper warning). It argued that the atomic bomb was no longer necessary and that peaceful negotiations with a weakened Japan were now feasible. It also warned of a deadly arms race if more countries gained the technology to develop nuclear weapons.

Seventy scientists signed the petition, but administration officials made sure it never reached Truman’s desk. Oppenheimer and members of Truman’s Interim Committee, which advised the administration on if, when, and how the bombs should be used, believed the weapons were the only way to quickly end the war. Ultimately, it decided not to formally warn the Japanese government of the incoming strike and to use the U.S.’s possession of nuclear weapons to influence postwar geopolitics, particularly in dealings with the Soviet Union under Stalin’s leadership. 

The Allies warned the Japanese people of impending destruction.

Shortly after the Potsdam Declaration was released on July 26, 1945, the U.S. began dropping leaflets warning of a potential attack over Japanese cities like Kyoto, Nagasaki, and Yokohama, calling for civilians to evacuate and petition their government to surrender. They provided generic, unspecific warnings so they could catch whichever targets they selected off guard to achieve maximum effect. 

The first set of leaflets did not explicitly state the U.S.’s intention to use an atomic bomb. In the weeks leading up the bombings, the U.S. purposefully did not carry out any traditional attacks on Hiroshima and Nagasaki in order to properly gauge the scope of the atomic weapons’ destruction. Despite the warnings, Japanese residents of Hiroshima and Nagasaki were accustomed to ongoing air raids, which may have led them to discount the danger they were in. 

After “Little Boy” was dropped on Hiroshima on August 6, the U.S. dropped a second round of leaflets over places like Tokyo, Osaka, and Fukushima, threatening destruction by a second atomic bomb. The second set of leaflets did make direct reference to a nuclear attack, but they were distributed over Nagasaki on the day of the actual bombing, giving people little time to evacuate. 

The Atomic Bombings of Hiroshima and Nagasaki: A Timeline of Key Events

The atomic bombs dropped on Hiroshima and Nagasaki were slightly different.

The United States deployed two types of nuclear weapons. “Little Boy” was a gun-type fission bomb using enriched uranium. When it was deployed, a gun mechanism inside the bomb shot fissile material into a target of the same fissile material, initiating a nuclear explosion. “Fat Man” was a more complex implosion-type fission bomb using plutonium, similar to the “Gadget,” the bomb detonated in the Trinity Test. 

On August 6, 1945, the Enola Gay B-29 Superfortress released “Little Boy” over Hiroshima, a city of about 300,000 people and large military installations. The bomb contained the explosive power of about 15,000 tons of TNT. Three days later, the B-29 Bock’s Car dropped “Fat Man” on Nagasaki, a major Japanese port, with the equivalent of 21,000 tons of TNT.

Nagasaki wasn’t the original second target.

After Hiroshima, the Americans’ next target was intended to be Kokura on the island of Kyushu, a city with a large military factory producing missiles, firearms, and chemical weapons for the Japanese army. The bombers were instructed not to deploy the weapon until they could get clear visibility of the factory. As the planes circled, clouds hid the city and eventually forced the bombers to their backup destination, Nagasaki—and the entire city of Kokura was spared by some cloud cover. 

When the bombers arrived in Nagasaki, visibility was partly obscured, but it wasn’t enough to stop the attack. 

Ironically, Kokura was also the backup target for the August 6 bombing of Hiroshima. Had that city been similarly covered by clouds, the American bombers would have gone to Kokura that day instead. 

Most sources claim between 110,000 and 210,000 people were killed in Hiroshima and Nagasaki.

The unparalleled destruction of Hiroshima and Nagasaki was, and is, almost unimaginable. “On the ground, eyewitnesses were largely unaware that it had been a single attack, and a consistency across accounts is their shock at realizing that the entire city had been affected at once by a single plane,” nuclear historian Alex Wellerstein wrote in the Bulletin of the Atomic Scientists in 2020. Wellerstein argued that it is nearly impossible to arrive at a completely accurate count of both bombings’ victims due to the nature of wartime record-keeping and displacement. U.S. military accounts at the time tended to emphasize the necessity of the bombings and underplay the suffering, and counted 110,000 people killed (70,000 in Hiroshima, 40,000 in Nagasaki). Reports in the 1970s from Japanese and anti-nuclear scientists estimated the number was at least 210,000.

At least a half-million survivors of the blasts, or hibakusha (“bomb-affected people”), experienced severe injuries and increased risks for cancer, birth defects, and fertility issues. More than 160 people (niju hibakusha, “double survivors”) experienced both bombings.

Survivors faced hardship and discrimination.

Along with chronic conditions stemming from or exacerbated by radiation exposure, many hibakusha faced discrimination socially and professionally. Some hibakusha found their marriage prospects diminished because potential partners thought survivors carried diseases or genetic anomalies. Survivors from designated zones were entitled to government benefits such as free healthcare, but some feared registering themselves or their children as hibakusha because of the stigma.

As of March 2025, hibakusha holding victim certificates (issued by the government beginning in 1957) numbered 99,130, and their average age is 86.

Approximately 10 to 20 percent of the victims were Korean forced laborers.

While the Japanese loss of life during the bombings was extraordinary, 10 to 20 percent of those killed in the bombings were Korean. After Japan colonized Korea in the early 20th century, more than 2 million Koreans were expatriated to Japan and over 750,000 were forced into labor by the Japanese colonizers. Many of the Koreans killed in Nagasaki and Hiroshima were forced laborers working in munitions factories to shore up the Japanese military. 

After the bombings and Japanese surrender, its occupation of Korea ended and some Korean survivors returned home. Little medical care was offered to them and many were left to suffer chronic conditions without the treatment their Japanese counterparts received. Some survivors expressed shock when Koreans applauded the bombings, since they effectively freed them from Japanese rule. Much like the hibakusha, Korean survivors faced discrimination upon their return, further compounding the impact of the bombings. 

The bombings weren’t the only reason Japan surrendered to the Allies.

The bombings remain one of the most controversial American military actions in history. While it’s been widely reported that the dropping of the atomic bombs on Hiroshima and Nagasaki diminished Japan to the point of surrender, many historians and high ranking officials in Truman’s administration postulated that war likely could have been ended without the use of nuclear weapons. Joseph Grew, who served as under secretary of state and the U.S. ambassador to Japan from 1932 to 1941, advocated for peaceful negotiations with Japanese leaders, believing Japan would surrender if the Allied forces allowed them to retain their emperor, Hirohito. 

Additionally, in the three days between the bombings, the Soviet Union invaded Manchuria—a long-contested part of present-day northeastern China then controlled by Japan—and officially declared war on Japan. The move quashed their hopes the Soviets would remain neutral and help negotiate favorable terms between the Japanese and Allied Forces. Long before the bombings, even Emperor Hirohito considered surrendering to the Allied forces to avoid a total destruction of the country. Japan’s growing fatigue in the war, coupled with the Soviet declaration of war and the destruction of Hiroshima and Nagasaki, ultimately led to Japan’s unconditional surrender and subsequent Allied military occupation. 

Einstein deeply regretted his role in kickstarting the Atomic Age.

The acclaimed physicist said the Einstein-Szilard letter was the “one great mistake” of his life. While Einstein’s role in atomic weapons development was minimal in comparison to figures like Oppenheimer, he felt he had opened Pandora’s Box by bringing the subject to the president.

Einstein said later that had he known Nazi Germany would never succeed in developing its own nuclear weapons, he would have said nothing on the matter. Following the destruction of  Hiroshima and Nagasaki, Einstein dedicated much of his public life to advocating for global nuclear disarmament.

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