It probably is apocryphal that scientists were not sure if the atmosphere would catch fire in a chain reaction when they would ignite the first nuclear explosion. It did not.
2 bombs were used in a war. 2 because there were two competing technologies, and the idea was to test both of them. Out of todays perspective it is strange that the US did not just demonstrate a bomb somewhere for the world to see without killing all those people. Around 100,000 people died in each incident. About the same died in Tokyo from firebombs. But many more airplanes were involved.
It is noteworthy that mankind managed to control this part of their capabilities so successful. When the Soviet Union dissolved I felt that there would be a 50/50 chance that the weapons would fall in the wrong hands. Even Israel and the apartheid regime in South Africa had them at some point.
In 2020 I would think that enough submarines with ICBMs would be beneficial for the European Union: They are the perfect deterrent, and would alter the need for US ‘protection’ from a Russia that starts talking about reparations for WW2 on its political fringes.