This post, from Phil Plait, is a must-read on the history of one of the most dangerous nuclear tests in history. I do have on quibble however, with the opening paragraph:
In 1958, the Soviet Union called for a ban on atmospheric tests of nuclear weapons, and went so far as to unilaterally stop such testing. Under external political pressure, the US acquiesced. However, in late 1961 political pressures internal to the USSR forced Khrushchev to break the moratorium, and the Soviets began testing once again. So, again under pressure, the US responded with tests of their own.
That’s a generous reading of the history. It could be argued, as Robert Heinlein said at the time, that the history was more like this: In 1958, the Soviet Union called for a ban on atmospheric test of nuclear weapons, and went so far as to unilaterally stop such testing. Of course, they had a massive country with closed borders where they could test weapons on the sly. The US eventually caved into to Soviet bullying and internal Communist sympathizers to join the ban. However, as Heinlein predicted, Khruschev later resumed testing when it suited him.