In August 1945, the first atomic bomb was dropped on the Japanese city of Hiroshima by the American B29 bomber Enola Gay. It exploded with the force of 20,000 tons of TNT. Everything within a mile of the centre was flattened.
The Americans dropped a second atomic bomb, codenamed 'Fat Man', on the Japanese city of Nagasaki.
The power of this new type of weapon meant that whichever country was in possession of it was stronger than any other country.
President Truman did not inform Stalin that the atomic bomb was going to be used. This created mistrust.
The USA wanted to show that it was stronger than the USSR, and vice-versa.
This competitive nature led to an arms race - not only to become the strongest, but also the most able and intelligent.