Four years after the US dropped two atomic bombs on Japan, the Soviet Union detonated its first atomic bomb on Aug. 29, 1949, setting the stage for an intensive nuclear arm race between the two superpowers.
The race was temporarily halted when the US and the Soviet Union, together with the UK — the third country with nuclear weapons — reached a moratorium on nuclear testing at the end of 1958.
Once the moratorium was lifted in 1961, nuclear tests peaked in 1962 with 178 tests detected in just one year.
The US and the Soviet Union then spent more than 20 years expanding their nuclear arsenals. The global stockpile of nuclear warheads reached its highest point in 1986 with more than 60,000 warheads excluding retired warheads.
Both nuclear tests and nuclear stockpiles were significantly reduced after the end of Cold War in 1991. In 1996, the Comprehensive Nuclear-Test-Ban Treaty, which bans nuclear explosions, was opened for signature. After 1996, only a handful of nuclear tests were conducted — by Pakistan, India and North Korea.