How does the Galaxy handle it when someone nukes the planet, a city or a country out of existence here on earth?

 This might be interesting to you. It is accomplished by retroactive time travel by the Galaxy.

The goal becomes tracking down who did it first and preventing the births of the terrorist or terrorists. Usually or often the mother of the terrorist is sterilized so she can have no more children either. This is just generally what actually happens. Sometimes, this is all that is necessary which is to go back and sterilize the mother of the nuclear terrorist. Other times, someone else will take that terrorists place in time and maybe 10 to 100 mothers will be sterilized before the terrorists are born in order to stop the nuclear blast or blasts from actually happening.

This is how it is usually done. So, first they have to track who actually did this thing. Then by preventing the births of the terrorists the nuclear blast or blasts which destroy cities or countries or the asteroid belts of Earth are stopped by sterilizing the mother before she gets pregnant and likely will never be able to get pregnant again.

This is how the galaxy handles cities or countries or the whole world blowing up in nuclear blast(s).

This has been done presently at least 100 times since 1935 or 1940 here on earth.

Why did they leave Hiroshima and Nagasaki as actually having happened?

As a warning to the world on into the future!