Throughout the Cold War immediately following the end of World War II, the world tested a mind-boggling number of nuclear weapons.
Although the Soviet Union and the US were the primary testers of nuclear weapons, France, the UK, China, India, Pakistan, and North Korea have all also tested nuclear devices at some point since the end of World War II.
However, peak testing of nuclear weapons in the atmosphere - the most potentially dangerous type of nuclear testing, as nuclear fallout could easily be dispersed by wind currents - occurred between 1961 to 1962 almost exclusively between the US and the USSR.
The following GIF, taken from a time-lapse map by Isao Hashimoto, shows the nuclear tests during that period:
During this period of testing, the USSR and the US detonated 340 megatons of nuclear weapons between the two of them. To put that in perspective, between 1945 and 1971 only 500 megatons of nuclear yield were detonated in total between all nuclear powers.
You can watch Hashimoto's entire time-lapse of nuclear weapons testing between 1945 and 1998 below: