Well if they're only an upstart, they don't have the ability to destroy you _yet_. You 'nuke' them in the hope they won't get that ability. You're aiming to stop MAD from being a thing.
In those terms, the US should have been nuking and dominating everyone, and the idea was floated after WW2, but I believe they were precluded by practical limitations.
If they had developed the tech outside of wartime, and built up a stockpile, maybe that is indeed what would have happened and we'd have a one-world government already.
Point is you cannot know if they are an upstart (whatever upstart means). It can be misinterpretation, it can be camoflage, it can be anything. But once you rain death you're better be prepared to be grateful for what you are about to receive back.
The dynamics at least with the space dark forest is different than when we live on the same planet. It has to do with lack/slow communication over vast space (that you can't trust anyway).
It relied on two principles "the chain of suspicion" and "technological explosion", which don't hold true if we are on the same planet. You can google it (or llm it) :)