The title is intentionally provocative, but let me try to justify it.
Let's take a common science fiction setup of two interplanetary civilizations with a hand-waved FTL technology that are at war (something involving either warp-bubble or hyperspace so that FTL travel doesn't involve relativistic speeds which causes even worse narrative problems for space war). Nearly all of their population live on planets or moons, with a small fraction living on space ships or space stations. Naturally, warfare focuses on the planets.
In common science fiction, an attack on a planet is done through roughly the process that Twilight Imperium uses to resolve combat:
- Have a space battle in around the planet to establish orbital superiority
- (Optional) bombard military forces on the planet
- Send down ground troops
But wait! Planets are at the bottoms of huge gravity wells. You don't need to shatter the planet into fragments to effectively destroy it. You just need to drop enough stuff into it's atmosphere to render it uninhabitable.
For example, the entirely of humanity's current nuclear arsenal (2022) has an estimated yield of 3 gigatons of TNT. The 10km asteroid that made the Chicxulub crater (and likely killed the dinosaurs) had an impact energy of 100,000 gigatonnes of TNT. Even a 2km or 3km asteroid could do the trick, and at that point, we're looking at the likely mass of some of the larger scale spaceships in science fiction.
But this leaves us in a predicament. If you have such large spaceships, you don't even need to establish orbital superiority to blow up a planet. First, you can start towing big asteroids into collision courses with the target planets. If they have defenses for massive asteroids, just show up in orbit with one of your super massive battlecruisers; if you're outnumbered in battle, you can crash it into the planet. If your enemy blows it into pieces, then the pieces still crash into the planet and impart a similar amount of kinetic energy.
Given this, it seems like it is much easier to destroy a planet than to assault it (a similar argument is presented in this video). There's a good discussion for why a faction would want to assault a planet rather than just blow it up here, but it seems to me that logic would only apply to the faction that is stronger/winning and has the resources to establish orbital superiority and assault the planets of the other. The faction that's weaker/losing just wants to not lose, which they could achieve by striking at the industrial/population/economic centers of the other faction by blowing up their planets.
So, given this setup, how does warfare not lead to the total annihilation of both sides? I see two potential angles here:
- There is some conceivable technology that makes blowing up planets hard and this starts to look more like a "conventional" space war. In this case, what is it (and why is there not also some conceivable technology that beats it)?
- One faction will get blown up but not the other. In this case, at the point at which one faction has started smashing planets, why does the other not respond in kind?
Some answers I want to rule out:
Mutually assured destruction doesn't help if war has already broken out.
A space Geneva convention requires some sort of international community working together such that there are other parties present to prosecute violators. It doesn't work if there are just two civilizations.