Today in 1949, the Soviet Union tested its first atomic bomb in Semipalatinsk, Kazakhstan. The 22-kiloton RDS-1 device—codenamed Joe-1 by the United States—was basically a copy of the first plutonium-fueled implosion-type atomic bomb (the “Gadget”) the US tested on July 16, 1945. 
Radioactive debris from this test was detected in early September by USAF WB-29 reconnaissance aircraft and a US Navy program to collect and analyze fallout in rainwater. Once traced back to its source (journals.ametsoc.org/view/journals/…), President Truman announced the test on Sept. 23.
Truman was initially not certain he wanted to publicly announce the Soviet nuclear test, worrying that doing so would create diplomatic and economic problems. When he finally did so, however, he did not say it was an atomic bomb test, only that “an atomic explosion occurred.”
Today is also the 30th anniversary of the closure of the Semipalatinsk Test Site—also known as the Polygon and the 12th International Day Against Nuclear Tests, established by the United Nations in 2009 at the urging of Kazakhstan and other states to commemorate that milestone.
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