From 1958 to 1961, as the world powers negotiated a treaty to ban all testing of nuclear weapons except underground, there was a major push in both the U.S. and the Soviet Union to complete as much development and testing of nuclear weapons as possible before such work was prohibited.
However, in September, 1961, the Soviets broke the moratorium, performing 45 nuclear test events in two months, culminating with a 60-megaton blast on October 30, 1961, the largest at the time.
Congressional ratification of the treaty was tied to a mandate to the U.S. Joint Chiefs of Staff to develop a program to maintain American readiness to resume testing if necessary.
[1] While flying simulations for the Test Readiness Program, the science teams assigned to the NC-135 aircraft realized that their flying laboratories could be effectively used to study solar eclipses, cosmic rays entering the atmosphere and the effects of magnetic fields in the ionosphere.
[2] By 1974, the world politics had significantly changed, and it was clear that the treaty, now over a decade old, would hold and there was little possibility of atmospheric testing would resume.