It was in 1949 that the Soviet Union exploded an atomic bomb, and the communists gained control of China. Both of these events were worrisome to the U.S.
USSR
When the Soviet Union exploded an atomic bomb, the US response was to increase its own arsenal and its own testing of nuclear weapons.
russians
The first exploded over Heroshima, the second over Nagasaki.
Pakistan
8.15 am
fartland
The first atomic bomb used against mankind exploded at 8.15 over Hiroshima.
j. rosenberg
See website: Atomic bombings of Hiroshima and Nagasaki. people living there died.
In short, the dropping of the Atomic Bombs showed the Russians we were "not afraid to use the big stuff" (phrase they used back then). Stalin got the idea he had to finish developing the atomic bomb. And, to the consternation of Winston Churchill, Truman let the Russians keep the Eastern European nations they defeated and have of East Germany. Winston Churchill had warned him this would spark a Cold War and that an Iron Curtain had fallen. Roosevelt and Truman had chosen not to tangle with Josef Stalin and the millions of communists of the USSR. Therefore, anti-communists blew all this out of proportion and they were very fearful.
It was a plutonium implosion type. So it was an atomic bomb.