USSR
The first hydrogen bomb tested by the US was codenamed "Mike" and tested in the Marshall Islands.
Any hostile nation with such a weapon puts us right back into the cold war; Tension.
Nobody, the US was the first country to work out how to actually make nuclear weapons.
The US performed the first nuclear weapon test on July 16, 1945.
End of WW2.
US
never, most of their nuclear weapon activity was for propaganda purposes.
US
They were pretty nervous...................
Both US & UK.
The US tested nuclear weapons on its own territory including the deserts of the continental southwest US.
Manhattan project