With a POD no earlier than 1900, when's the earliest that you could, A), possibly, and B), plausibly, have a nation develop and detonate the world's first nuclear warhead? Would it be both possible and plausible for any nation to develop the world's first nuclear weapon quicker and earlier than the USA did IOTL? How much would it stretch plausibility to, let's say, have the world's first nuclear warhead detonated successfully a decade or earlier than OTL's Trinity test (July 1935 or earlier)? And if not the USA, who else could potentially pull this off?