Essentially falls into the Goodwar/Badwar doctrine. Everyone knows you can do it, but nobody does because nobody wants a Black War. While I don't think there is anything like a "Fourth Protocol" against it (because there is no evidence of any sort of arms treaties between interstellar powers like that) I do think that there is a similar set of understandings about "how war will be conducted" between the various polities - and it is mainly that WMD's are off limits. The interesting thing is that on an Interstellar level nukes aren't really considered WMD's any more (given starship weapons, and meson artillery).
For non-state actors the definition of WMD broadens out to include nukes, and probably a couple of other things that we can't think of yet. Nanites maybe? But fundamentally, it's the non-state actors that really tend to produce the "Nightmare Scenarios" because they are not bound by conventional strategy and tactics.
For me, this stuff is gold.
Of course someone could try it. Of course, someone might pull it off. (As stated above, it might well have already been tried in the past.) I love the idea of politics in tension, that things can go wrong, that society and societies try their best to put the lid on the horrors humans are capable of -- but still, things go horribly wrong. (Because Life, People.)
The only use it has, of course, for RPG play, though, is how it manifests for the PCs. And this, as I said, is a goldmine.
Setting up such an attack takes time, takes communication. The mere fact that someone is trying such a think would be a Big Deal, which would generate buzz within the organization/state planning the attack. Which means that even before the attack is launched there are ways for PCs to get involved in adventure -- either tracking down clues, or trying to get a warning out to authorities if they stumble across the information by accident, or trying to stop it before the plan can go forward. If the plan is already underway, as other have noted, higher TL worlds will have defense systems in place. If such a world is a target efforts will have to be made to cripple those systems. These are only more opportunities for the potential attack to fail even before it begins and more opportunities for the PCs to get involved in gaining information/stopping such efforts. (After all, there will be
dozens of ways of striking that the planetary defense systems. What is the point of the attack for the aggressors? Once word gets out that
something is happening, someone will need to find out the specifics ASAP before the attack is executed.) Then it becomes cat and mouse (and, for me, screw ups in the bureaucracy that can't believe anyone would do such a thing, or interagency fighting, or moles within the security system and whatnot that only make the need of the PCs to sort it out even more vital.)
Broadening out we get wonderful questions: Why would someone want to do such a thing? I mean, you don't just blow up a nuke in a major city because you can. There's got a be a powerful (and in the case we're discussing here) REALLY POWERFUL motivation for committing such an act. The moment you go down this road your setting becomes richer, if only because you suddenly need to justify a conflict between the aggressor and the target world WORTHY of such a conflict. Now there is tension and politics and some sort trauma in the setting where conflict is guaranteed, with names and characters attached who really care about something and there will be consequences because of this.
That is awesome stuff for an RPG setting.
Moreover, lets say efforts to stop the attack fail. Horrible damage is done. Worlds get word this is happening. These worlds are terrified. As Quint properly suggested, there will be a moral/instinctual response, since this is the kind of thing
people do not do. A hunt to put down the group/state that committee the atrocity is on. Again, who are they? What are their resources? It it a rogue organization of a few a dozen to a few thousand zealous of one kind or another? Or is it a political state that will have to be defeated through overt violence or covert subterfuge?
All of this, again, means
adventure. The setting is in motion. Lives are at stake. Decisions must be made. Action must be taken. The clock is ticking.
Jesus... just typing this I'm all, "I want to a build a subsector just for this scenario!" with a hunt across the worlds to put down the mother▮▮▮▮▮▮ers who did this. Like,
right now.
This is some great stuff for story, setting, and RPG adventures.