As the Potsdam conference approached, the USA was clse to developing an effective atomic bomb. The first successful test took place on 16 July, the day before the Conference opened. The atomb bomb had been developed in complete secrecy. However, once Truman knew it worked, he told Stalin about it.
In face, Staling knew about the project all along- his spies had been keeping him informed since 1942.
On 6 August 1945, the USA dropped an atomic bomb on the Japanese city of Hiroshima. The blast killed around 140,000 people. Three days later the USA dropped a second atomic bomb on the Japanese city of Nagasaki, killing around 74,000 people They dropped the bombs to make the Japanese surrender but to also send a warning to the Soviets. and Stalin began creating his own.