By the summer of 1945, World War II had raged across several continents for six years, beginning when German forces invaded Poland in 1939.
To a world grown weary of death and destruction, the surrender of Nazi Germany on May 8, 1945, was welcome news. Japan, however, vowed to fight to the very end. Their resolve was evident in the Battle of Okinawa: By the time it ended in June, Allied forces lost more than 14,000 soldiers and Japan's military suffered more than 77,000 deaths — plus the loss of an estimated 100,000 Japanese civilians.
Against this backdrop, Allied forces drew up plans for Operation Downfall, a large-scale invasion of Japan. But with Allied casualties from an invasion estimated to reach 1 million deaths — plus another 10 million Japanese casualties, including civilians — Allied planners searched for another way to end the war, which they found in the Manhattan Project.
The Manhattan Project
Concerned over reports that Nazi Germany was developing a new type of weapon using radioactive uranium, in 1939 U.S. government officials began to investigate the potential of uranium — particularly the uranium-235 isotope — for making a powerful bomb.
Within a matter of months, a coalition of American, British and European scientists — many of whom were refugees from Germany, Italy and other fascist nations — began collaborating on a vast international project to develop a uranium-based bomb before any Axis powers beat them to it.
Because one early component of the project was based in the U.S. Army's Manhattan District, the name Manhattan Project eventually stuck, though work was carried out in Los Alamos, New Mexico; Oak Ridge, Tennessee; Hanford, Washington; Chalk River, Ontario, and several other sites.
After several years of intensive research under strict secrecy, the Manhattan Project developed two different bombs that used two different nuclear materials: uranium-235 and plutonium-239.
'Little Boy' and 'Fat Man'
About 140 pounds (64 kilograms) of highly enriched uranium-235 was used to create "Little Boy," a nuclear-fission bomb that worked by shooting a large, hollow cylinder of uranium over a smaller uranium insert. Far from little, the bomb weighed about 9,700 pounds (4,400 kg).
"Fat Man" was even bigger — about 10,300 pounds (4,670 kg) — and used a core of plutonium-239 to create a nuclear chain reaction that would release the energy of about 21 kilotons of TNT dynamite. [Countdown: Top 10 Greatest Explosions Ever]
The complexity of the plutonium bomb caused some concern among project engineers, so a test of the bomb was scheduled for July 16, 1945. On that day, at Alamogordo, New Mexico, the first atomic bomb blast the world had ever witnessed occurred, ushering in the Atomic Age.
In 1945, Hiroshima, Japan, was a city of about 350,000 people that was largely untouched by the war. Situated on a coastal plain, the city was an important manufacturing and military center — at least 40,000 military personnel were stationed in Hiroshima during the summer of 1945.
For these and other reasons, the city was selected as the first target of an atomic bomb attack. Early in the morning of Aug. 6, a B-29 airplane named Enola Gay (after the mother of its pilot, Colonel Paul Tibbets) took off from Tinian, an island roughly six hours from Hiroshima by air. Inside the Enola Gay's bomb bay was "Little Boy."
At 8:15 a.m., the Enola Gay's bay doors opened and "Little Boy" was dropped over Hiroshima. The bomb exploded about 1,900 feet (580 meters) over the unsuspecting city. The enormous blast instantly destroyed most of the city and claimed some 45,000 lives; another 19,000 people are estimated to have died from radiation exposure or other direct effects within four months.
"The mushroom [cloud] itself was a spectacular sight, a bubbling mass of purple-gray smoke and you could see it had a red core in it and everything was burning inside," said the Enola Gay's tail gunner, Staff Sgt. Robert Caron. "As we got farther away, we could see the base of the mushroom and below we could see what looked like a few-hundred-foot layer of debris and smoke."
On the ground, however, the scene was more horrific than spectacular. "In the following waves [after the initial blast] people’s bodies were terribly squeezed, then their internal organs ruptured," wrote a journalist with LIFE magazine. "Then the blast blew the broken bodies at 500 to 1,000 miles per hour through the flaming, rubble-filled air. Practically everybody within a radius of 6,500 feet was killed or seriously injured and all buildings crushed or disemboweled."
To the surprise of some Allied military commanders, the destruction of Hiroshima did not bring about a Japanese surrender. Of the cities remaining on the Allied force's list of potential targets, the ancient fortress city of Kokura was at the top of the list.
Shortly before 4 a.m. on Aug. 9, a B-29 named Bockscar (a pun on the aircraft's captain, Frederick C. Bock) took off from Tinian, headed for the city of Kokura. In the plane's bomb bay was the plutonium-fueled "Fat Man."
Cloud cover and poor visibility over Kokura spared the city, so the crew of Bockscar headed toward Nagasaki. A large seaport, the city was home to several important manufacturers, including the Mitsubishi Steel and Arms Works, and enterprises devoted to shipbuilding, weapons development and other military industries.
It, too, was obscured by clouds, but the crew was able to find a break in the cloud cover, and shortly after 11 a.m., "Fat Man" exploded over Nagasaki.
The powerful blast destroyed much of the city, and at least 40,000 people were killed immediately. The hilly terrain of Nagasaki — and the fact that the bomb was dropped almost 2 miles (3.2 km) from its intended target — prevented greater destruction.
Nonetheless, by the end of 1945, about 80,000 people died from the bomb over Nagasaki. About half of the city was reduced to rubble and ash by the blast and the subsequent fires.
Effects of the blasts
At noon on Aug. 15 — six days after Nagasaki was bombed — Japanese Emperor Hirohito announced an unconditional surrender, effectively ending World War II. Jubilant celebrations broke out across the globe, but there was little to celebrate in Japan.
The exact number of deaths from the bombings of Hiroshima and Nagasaki will never be known, but at least 103,000 died from the immediate blast or the effects of radiation sickness, according to the World Nuclear Organization.
There were also a high number of stillbirths and birth deformities reported among survivors of the bombings. However, no long-term genetic damage has been reported among the survivor's children, who are subject to ongoing screening.
After the victors' initial celebrations quieted down, there was profound soul-searching by most of the people involved in the decision to use atomic weaponry. The devastation wrought by two just two bombs was unprecedented in human history, and the debate over the use of nuclear weapons remains relevant to this day.
"Yesterday man unleashed the atom to destroy man, and another chapter in human history opened, a chapter in which the weird, the strange, the horrible becomes the trite and obvious," wrote a New York Times correspondent one day after the Hiroshima bombing.
Alarmed by the use of atomic weaponry, many of the scientists responsible for the Manhattan Project organized as the Federation of Atomic Scientists, an organization that continues to work against nuclear proliferation.