I'd have to research when the last time the US tested nuclear weapons.
Some of the history is that the early cold war had the US doing a lot of testing of bigger and bigger nuclear weapons. The US made the region around the Bikini Atoll uninhabitable. Russia set off an incredibly large atmospheric weapon and the dangers of the testing became clearer and clearer. The US agree to halt if Russia did, but Russia would then do another and the US would to match them.
This was the situation when JFK became president, with his top priority being to lower the hair-trigger situation for nuclear war Eisenhower had created, by reducing our conventional forces in Europe and adopting a doctrine that our first and only response to a conflict would be nuclear weapons, pushing MAD deterrence to an extreme and extremely dangerous level.
An anecdote about this is that as soon as JFK took office, for this effort, he ordered his Secretary of Defense, McNamara, to review our nuclear war plans; they had been created by the air force and were 'owned' by them and had never been seen by a civilian. The Air Force told their boss, McNamara, no, and he had to go to JFK to get a presidential order to the Air Force to show them to McNamara.
(When he saw them, they were basically targeting every big city in the Soviet Union and China; if there had been a conflict with the USSR leading to nukes, apparently they'd decided we should take out China also while we were at it.)
The second part of JFK's effort became the first cold war treaty to reduce nuclear weapon use, the limited atmospheric test ban treaty. This became JFK's top priority, but he faced a lot of opposition - his own Pentagon was going to the Congressional hearings to testify in opposition to his bill.
But JFK did a lot to get support for it and it finally passed, and JFK viewed that bill as his greatest achievement as president.
After that, testing went underground. I don't have data on the ongoing frequency of any US tests, but it's probably not hard to find. The largest bomb I mentioned, in 1961, was 10 times all munitions used in WWII; Hiroshima was 15 kilotons, it was 57,000 kilotons, visible over 1,000 kilometers away.
On October 30, 1961, the Soviet Union tested the largest nuclear device ever created. The "Tsar Bomba," as it became known, was 10 times more powerful than all the munitions used during World War II.
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