In August 1945, the first atomic bombs were dropped on the Japanese cities of Hiroshima and 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.
Stalin was suspicious of why he had not been informed. Stalin also realised that the new bombs could be used against the USSR as he did not have any.
The ideological differences between the USSR and the USA resulted in the desire for each country to outdo the other.
This competitive nature led to an arms race - not only to become the strongest, but also the most able and intelligent.