In August 1945, the US Air Force dropped two atomic bombs on the Japanese cities of Hiroshima and Nagasaki. They caused hundreds of thousands of civilian casualties and were shortly followed by the surrender of Japan and the end of World War 2.
Why is this important?
It was the first and only time that atomic bombs have been used in a war. Looking at what happened to these cities is probably the best way of understanding the death and damage that will be caused if there is another nuclear conflict.
Why were the bombs used?
Deciding whether to use the bombs was like calculating an equation. The Americans believed the number of people saved by ending World War 2 quickly would be greater than the number of people killed by the atomic bombs.
But there is still some controversy about whether this was true.
What damage did the bombs cause?
At Hiroshima, the blast flattened buildings within a 2.5 km radius of the bomb. At Nagasaki the hilly landscape meant the destruction was less widespread.
The bombs killed about 240,000 people. Around 120,000 were killed outright by the bombs, and a similar number died of injuries and radiation sickness in the weeks, months and years that followed.
What did the US hope to achieve?
Japan was at war with America and its allies, which included Britain and Russia. The allies were winning the war and the Japanese forces had been pushed back from many locations. However fighting was still very fierce and soldiers and civilians were dying every day.
US President Harry S Truman wanted the Japanese to surrender as quickly as possible so he could save lives.
The atomic bomb was a deadly new weapon. Truman hoped the massive destruction it caused would shock the Japanese into realising they had to surrender.
What did the US hope to avoid?
President Truman wanted to avoid a land invasion of Japan. There were 2.5 million Japanese troops stationed there and Truman's staff estimated that defeating them would cost the lives of 250,000 US soldiers.
Some historians also say that the US wanted to avoid Japan being occupied by Russian troops. America and Russia were allies but they did not really trust each other.