A mushroom cloud is seen over Nagasaki, Japan, after the United States dropped an atomic bomb on the city on August 9, 1945. It came three days after a bomb was dropped over Hiroshima, Japan.
In pictures: The bombings of Hiroshima and Nagasaki
A mushroom cloud is seen over Nagasaki, Japan, after the United States dropped an atomic bomb on the city on August 9, 1945. It came three days after a bomb was dropped over Hiroshima, Japan.
Time Life Pictures/US Air Force/Getty Images

Updated 1411 GMT (2211 HKT) August 09, 2020

Seventy-five years ago, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki.

The devastation led to Japan's unconditional surrender and brought an end to World War II.

To this day, it is the only time that nuclear weapons have been used in warfare.

Editor's note: This gallery contains graphic images. Viewer discretion is advised.

In 1939, physicists Albert Einstein and Leo Szilard drafted a letter to US President Franklin D. Roosevelt, urging him to research atomic bombs before the Germans could build one first. By 1942, the United States had approved the top-secret Manhattan Project to build a nuclear reactor and assemble an atomic bomb.<br /><br />Here, calutron operators work in Oak Ridge, Tennessee, in 1944. Calutrons were used to refine uranium ore into fissile material.
In 1939, physicists Albert Einstein and Leo Szilard drafted a letter to US President Franklin D. Roosevelt, urging him to research atomic bombs before the Germans could build one first. By 1942, the United States had approved the top-secret Manhattan Project to build a nuclear reactor and assemble an atomic bomb.

Here, calutron operators work in Oak Ridge, Tennessee, in 1944. Calutrons were used to refine uranium ore into fissile material.
Galerie Bilderwelt/Hulton Archive/Getty Images
A billboard in Oak Ridge reminds workers of the top-secret nature of the facility.
A billboard in Oak Ridge reminds workers of the top-secret nature of the facility.
Ed Clark/The LIFE Picture Collection/Getty Images
Scientists with the Manhattan Project manually haul a container of radioactive material from a shed in Los Alamos, New Mexico.
Scientists with the Manhattan Project manually haul a container of radioactive material from a shed in Los Alamos, New Mexico.
Los Alamos National Laboratory/The LIFE Picture Collection/Getty Images
Lie-detection tests were administered as part of security screening for the Manhattan Project.
Lie-detection tests were administered as part of security screening for the Manhattan Project.
Galerie Bilderwelt/Hulton Archive/Getty Images
These photos show some of the people who worked on the Manhattan Project at the Los Alamos National Laboratory in New Mexico. US Army Col. Leslie R. Groves, second row on the left, was appointed to head the project. Physicist J. Robert Oppenheimer led the lab at Los Alamos.
These photos show some of the people who worked on the Manhattan Project at the Los Alamos National Laboratory in New Mexico. US Army Col. Leslie R. Groves, second row on the left, was appointed to head the project. Physicist J. Robert Oppenheimer led the lab at Los Alamos.
Everett/Shutterstock
The United States detonated the world's first atomic bomb at a test site in New Mexico on July 16, 1945. Trinity was the code name of the test bomb, which was detonated in the Jornada del Muerto desert.
The United States detonated the world's first atomic bomb at a test site in New Mexico on July 16, 1945. Trinity was the code name of the test bomb, which was detonated in the Jornada del Muerto desert.
CORBIS/Getty Images
Groves and Oppenheimer examine twisted wreckage after the test blast. This is all that remained of a 100-foot tower, winch and shack that held the nuclear weapon.
Groves and Oppenheimer examine twisted wreckage after the test blast. This is all that remained of a 100-foot tower, winch and shack that held the nuclear weapon.
CORBIS/Getty Images
Air Force Col. Paul Tibbetts waves from the pilot's seat of the Enola Gay moments before takeoff on August 6, 1945. A short time later, the plane's crew dropped the first atomic bomb in combat.<br /><br />Eight days earlier, US President Harry Truman had warned Japan that the country would be destroyed if it did not surrender unconditionally.
Air Force Col. Paul Tibbetts waves from the pilot's seat of the Enola Gay moments before takeoff on August 6, 1945. A short time later, the plane's crew dropped the first atomic bomb in combat.

Eight days earlier, US President Harry Truman had warned Japan that the country would be destroyed if it did not surrender unconditionally.
Armen Shamlian/US Army Air Forces/LIFE/Getty Images
A mushroom cloud billows over Hiroshima about one hour after detonation. At least 70,000 people were killed in the initial blast, while approximately 70,000 more died from radiation exposure. "The five-year death total may have reached or even exceeded 200,000, as cancer and other long-term effects took hold," according to the Department of Energy's <a href="https://www.osti.gov/opennet/manhattan-project-history/Events/1945/hiroshima.htm" target="_blank" target="_blank">history of the Manhattan Project.</a>
A mushroom cloud billows over Hiroshima about one hour after detonation. At least 70,000 people were killed in the initial blast, while approximately 70,000 more died from radiation exposure. "The five-year death total may have reached or even exceeded 200,000, as cancer and other long-term effects took hold," according to the Department of Energy's history of the Manhattan Project.
US Army/Hiroshima Peace Memorial Museum/AP
A patient suffering severe radiation burns lies in the Hiroshima Red Cross hospital in August 1945. Many of those who survived the initial blast died of severe radiation-related injuries and illnesses.
A patient suffering severe radiation burns lies in the Hiroshima Red Cross hospital in August 1945. Many of those who survived the initial blast died of severe radiation-related injuries and illnesses.
Hajime Miyatake/Asahi Shimbun/Getty Images
This aerial photo shows the Japanese Army base at Hiroshima, a day after the bomb.
This aerial photo shows the Japanese Army base at Hiroshima, a day after the bomb.
Bettmann/Getty Images
Bomb survivors lie in bed in Hiroshima.
Bomb survivors lie in bed in Hiroshima.
Universal History Archive/Getty Images
A person's outline is seen on bank steps after the bombing in Hiroshima.
A person's outline is seen on bank steps after the bombing in Hiroshima.
Universal History Archive/Getty Images
A man walks through the destruction in Hiroshima.
A man walks through the destruction in Hiroshima.
Universal History Archive/Getty Images
A worker stands next to an atomic bomb, code name "Fat Man," hours before it was dropped on Nagasaki, Japan, on August 9, 1945. The Hiroshima bomb was nicknamed "Little Boy."
A worker stands next to an atomic bomb, code name "Fat Man," hours before it was dropped on Nagasaki, Japan, on August 9, 1945. The Hiroshima bomb was nicknamed "Little Boy."
Prisma Bildagentur/UIG/Getty Images
This photo was taken about six miles from the scene of the Nagasaki explosion. According to the Nagasaki Atomic Bomb Museum, photographer Hiromichi Matsuda took this 15 minutes after the attack.
This photo was taken about six miles from the scene of the Nagasaki explosion. According to the Nagasaki Atomic Bomb Museum, photographer Hiromichi Matsuda took this 15 minutes after the attack.
Universal History Archive/Getty Images
Bones and ashes are left behind after the bombing of Nagasaki, which killed up to 80,000 people.
Bones and ashes are left behind after the bombing of Nagasaki, which killed up to 80,000 people.
Eiichi Matsumoto/Asahi Shimbun/Getty Images
A woman and a child walk in Nagasaki on the day of the bombing.
A woman and a child walk in Nagasaki on the day of the bombing.
STR/Associated Press
An aerial view of Nagasaki after the bombing.
An aerial view of Nagasaki after the bombing.
Bettmann/Getty Images
A young survivor cries as he receives treatment at a temporary hospital in Nagasaki.
A young survivor cries as he receives treatment at a temporary hospital in Nagasaki.
Yasuo Tomishige/Asahi Shimbun/Getty Images
People in Osaka, Japan, listen to a radio broadcast of Emperor Hirohito on August 15, 1945. He announced that Japan had accepted the terms of the Potsdam Declaration, which called for the country's unconditional surrender.
People in Osaka, Japan, listen to a radio broadcast of Emperor Hirohito on August 15, 1945. He announced that Japan had accepted the terms of the Potsdam Declaration, which called for the country's unconditional surrender.
Asahi Shimbun/Getty Images
US President Harry Truman announces Japan's surrender to media assembled at the White House.
US President Harry Truman announces Japan's surrender to media assembled at the White House.
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A jubilant American sailor kisses a nurse in New York's Times Square as he celebrates the news that Japan has surrendered.
A jubilant American sailor kisses a nurse in New York's Times Square as he celebrates the news that Japan has surrendered.
Alfred Eisenstaedt/The LIFE Picture Collection via Getty Images
Soldiers and sailors on the USS Missouri watch as Japan's formal surrender is signed in Tokyo Bay on September 2, 1945.
Soldiers and sailors on the USS Missouri watch as Japan's formal surrender is signed in Tokyo Bay on September 2, 1945.
Carl Mydans/The LIFE Picture Collection/Getty Images