6 Things To Know About The Atomic Bomb

The giant purple mushroom cloud boiled 45,000 feet high over the Japanese city of Hiroshima. The death and destruction left in its wake were unlike anything the world had ever seen: 90 percent of the city wiped out in the blink of an eye.

The U.S. dropped the world’s first nuclear bomb on Hiroshima on August 6, 1945, and then, three days later, a second bomb on Nagasaki. Those bombings 75 years ago forced Japan to surrender, bringing an end to World War II (1939-45).

But the cost was massive. The combined death toll was estimated to be between 129,000 and 226,000 Japanese citizens—mostly civilians. The bombings also changed warfare and national security forever. They ushered in the atomic age, which has made the world a more complex and dangerous place, as nuclear weapons have spread around the globe.

From schoolchildren crouching under their desks in radioactive fallout drills during the Cold War to nuclear threats from North Korea today, the world hasn’t been the same since.

Here are six important things to know about the atomic bombings that changed history.

The giant purple mushroom cloud shot up 45,000 feet high over the Japanese city of Hiroshima. The death and destruction left in its wake were unlike anything the world had ever seen. The blast had wiped out 90 percent of the city in the blink of an eye.

The U.S. dropped the world’s first nuclear bomb on Hiroshima on August 6, 1945. Three days later, American forces dropped a second bomb on Nagasaki. Those bombings 75 years ago forced Japan to surrender, bringing an end to World War II (1939-45).

But the cost was massive. The combined death toll was estimated to be between 129,000 and 226,000 Japanese citizens. Most of them were civilians. The bombings also changed warfare and national security forever. They ushered in the atomic age, leading to nuclear weapons being spread around the globe. It’s made the world a more complex and dangerous place.

The world hasn’t been the same since. During the Cold War, schoolchildren crouched under their desks in radioactive fallout drills. Even today, North Korea continues to make nuclear threats.

Here are six important things to know about the atomic bombings that changed history.

On July 16, 1945, members of the Manhattan Project and the U.S. Army waited in the rain in the New Mexico desert for “Trinity.“ That was the code name for the first secret test of a nuclear bomb. At 5:29 a.m., the detonated bomb produced a massive explosion equal to 20,000 tons of dynamite. The test was a success. In fact, it went far beyond the original calculations of the bomb’s potential power.

But there was something else the scientists didn’t expect: that the radiation from those tests would be fatal. Radiation is damaging to dividing cells. The massive nuclear fallout sparked an increase in the death of infants. It also led to the development of certain kinds of cancers in the U.S. communities caught in the fallout’s path. Known as “Downwinders,” residents of those towns were the first casualties of the atomic arms race.

In 1990, the U.S. government officially apologized to the 22,220 Downwinders. Federal officials awarded them more than $2 billion in compensation. The settlement didn’t apply to their descendants.

By the summer of 1945, Germany had surrendered, but Japan remained the largest Axis power still waging war against the U.S. and its allies, including Britain and the Soviet Union. President Roosevelt had died on April 12, 1945, thrusting Vice President Harry Truman into the commander-in-chief role.

The U.S. was faced with some particularly dark math: Drop atomic weapons on largely civilian populations and force an end to the war or press on with a conventional war and costly invasion of Japan that would perhaps mean the loss of hundreds of thousands more American soldiers. The last major battle on Okinawa had claimed the lives of more than 150,000 American and Japanese soldiers.

On August 6, a U.S. warplane dropped an atomic bomb nicknamed Little Boy over Hiroshima. The explosion destroyed 5 square miles of the city. Some 80,000 people were killed instantly.

Kimura Yoshihiro, a third-grader at the time, saw the bomb fall.

“Five or six seconds later, everything turned yellow,” he later recalled. “It was like I’d looked right at the sun. Then there was a big sound a second or two later and everything went dark.”

By the summer of 1945, Germany had surrendered. That left Japan as the largest Axis power still waging war against the U.S. and its allies, including Britain and the Soviet Union. President Roosevelt had died on April 12, 1945. His death propelled Vice President Harry Truman into the commander-in-chief role.

The U.S. was faced with some particularly dark math: Drop atomic weapons on largely civilian populations and force an end to the war or press on with a conventional war and costly invasion of Japan. That would perhaps mean the loss of hundreds of thousands more American soldiers. The last major battle on Okinawa had claimed the lives of more than 150,000 American and Japanese soldiers.

On August 6, a U.S. warplane dropped an atomic bomb nicknamed Little Boy over Hiroshima. The explosion destroyed 5 square miles of the city. Some 80,000 people were killed instantly.

Kimura Yoshihiro, a third-grader at the time, saw the bomb fall.

“Five or six seconds later, everything turned yellow,” he later recalled. “It was like I’d looked right at the sun. Then, there was a big sound a second or two later and everything went dark.”

The second A-bomb, dubbed Fat Man, was dropped on Nagasaki three days later, instantly killing between 35,000 and 40,000 people.

But what came after was also devastating. In both cities, tens of thousands died of burns and radiation poisoning, stretching years after the initial bombing, though exact figures have been debated. After the war, the U.S. tried to cover up the horrors caused by the radiation, censoring reports from Japan.

“Some U.S. leaders privately feared that if the true extent of the bombs’ aftermath in Hiroshima and Nagasaki were known . . . the U.S. might squander its hard-earned moral victory,” says Lesley M.M. Blume, author of Fallout: The Hiroshima Cover-Up and the Reporter Who Revealed It to the World.

The second A-bomb was dubbed Fat Man. It was dropped on Nagasaki three days later, instantly killing between 35,000 and 40,000 people.

But what came after was also devastating. In both cities, tens of thousands died of burns and radiation poisoning. These deaths stretched years after the initial bombing. And exact figures of how many died from these causes have been debated. After the war, the U.S. tried to cover up the horrors caused by the radiation, censoring reports
from Japan.

“Some U.S. leaders privately feared that if the true extent of the bombs’ aftermath in Hiroshima and Nagasaki were known . . . the U.S. might squander its hard-earned moral victory,” says Lesley M.M. Blume, author of Fallout: The Hiroshima Cover-Up and the Reporter Who Revealed It to the World.

As the casualty figures in Nagasaki rolled in, so did a telegram from General Leslie Groves, director of the Manhattan Project, informing Truman that another bomb could be ready in about a week.

Truman was horrified and ordered that all atomic bombing cease. He told his Cabinet that he didn’t like the thought of killing “all those kids.”

Up until then, the military had been largely in control of the atomic weapons program. Though Truman approved of using nuclear weapons on Japan, many of the major decisions, such as when and where to drop the bombs, were made by the military.

But after the war, Truman worried about what might happen if future use of nuclear weapons remained in the military’s hands. So in 1948, he transferred control of them to the office of the presidency. Nuclear weapons were used, he said, “to wipe out women and men and unarmed people. . . . So we have got to treat this differently from rifles and cannon and ordinary things like that.”

That decision changed the U.S. nuclear program going forward. Before then, there was no formal policy on who could authorize the use of nuclear weapons. Today, the authority to use them rests with the office of the presidency.

As the casualty figures in Nagasaki rolled in, so did a telegram from General Leslie Groves, director of the Manhattan Project. He informed Truman that another bomb could be ready in about a week.

Truman was horrified and ordered that all atomic bombing stop. He told his Cabinet that he didn’t like the thought of killing “all those kids.”

Up until then, the military had been largely in control of the atomic weapons program. Truman approved of using nuclear weapons on Japan. But many of the major decisions, such as when and where to drop the bombs, were made by the military.

But after the war, Truman worried about what might happen if future use of nuclear weapons remained in the military’s hands. So in 1948, he transferred control of them to the office of the presidency. Nuclear weapons were used, he said, “to wipe out women and men and unarmed people. . . . So we have got to treat this differently from rifles and cannon and ordinary things like that.”

That decision changed the U.S. nuclear program going forward. Before then, there was no formal policy on who could approve the use of nuclear weapons. Today, the authority to use them rests with the office of the presidency.

Using A-bombs, even at the end of World War II, was controversial, and Truman forever defended his actions. In a letter to his sister, he later wrote, “I made the only decision I ever knew how to make. I did what I thought was right.”

But others thought bombing civilian populations with nuclear weapons was an inhuman, immoral act.

Popular opinion has shifted over time. In 1945, about 85 percent of Americans approved of the bombings, according to a Gallup poll. By 2015, however, only 56 percent of Americans thought the use of atomic weapons was “justified.”

Using A-bombs was controversial, even at the end of World War II. Still, Truman always defended his actions. In a letter to his sister, he later wrote, “I made the only decision I ever knew how to make. I did what I thought
was right.”

But others thought bombing civilian populations with nuclear weapons was an inhuman, immoral act.

In 1945, about 85 percent of Americans approved of the bombings, according to a Gallup poll. But popular opinion has shifted over time. By 2015, only 56 percent of Americans thought the
use of atomic weapons was “justified.”

The bombings ushered in a new era: the atomic age. By 1949, the Soviet Union also had nuclear weapons. No longer an American ally, the communist state began an arms race with the U.S. (see timeline slideshow, below).

Today, nine nations have nuclear arsenals (see “Today’s Nuclear Club,” p. 19). These weapons remain at a ready-for-launch status. There have been recent escalations in North Korea’s and Iran’s nuclear programs. That‘s increased the concerns about a future nuclear accident or conflict.

Ever since the U.S. bombed Japan, peace activists have led a movement to try to abolish nuclear weapons. Many people argue that it’s unlikely countries will give up their nukes. But others say it’s necessary to build a safer world.

“Nuclear weapons threaten our security more than protect us,” says former Secretary of Defense William J. Perry, who has co-written a new
book about the bombings, called The Button. “I would hope this generation would find a way to get rid of nuclear weapons.”

Robert K. Elder is the Chief Digital Officer for the Bulletin of the Atomic Scientists.

Robert K. Elder is the Chief Digital Officer for the Bulletin of the Atomic Scientists.