The idea that war is in any way supposed to be civil and countries/people are supposed to hold back is kind of fucking retarded IMO
If you're at war with someone the point is to end the war as quickly as possible preferably with the capitulation of the other side, and whether that capitulation comes from using diplomatic channels to make them realize they're absolutely fucked and would be better off surrendering, or going full Mongol and mercilessly butchering planets full of people until the other side gets the fucking picture is largely irrelevant. Nothing about war is civil, nothing about war implies there are rules that need to be followed, everything in war is a matter of what is more likely to bring the war to a swift close with minimal casualties on your side and a victory condition and honestly nuking the everloving shit out of your enemy from orbit beyond their ability to even retaliate, and doing it in such rapid succession that they are immediately on the back foot and begging you to stop is pretty fucking sound from every standpoint but a humanitarian one and if you give a good god's damn about humanitarian behavior in a war you're fucked in the head.
Of course the Blakists were goddamn retarded about how they did it, but the Clans have always been and will always be fucking morons who romanticize and exalt something that is supposed to be terrible, preferably brief and hopefully seldom. Make war terrible enough, people start treating it as a serious goddamn problem and start doing their damnedest to avoid it altogether instead seeing it as an opportunity to gloryhound and collect commendations or plunder stuff or throw political enemies into the meatgrinder. War should not stop until it comes right to the doorstep of a nation's leadership and burns their house down with them inside.
You are correct under the assumption that it's an isolated 2-party struggle and one side can overwhelm the other so quickly and so conclusively that it never has to fear reprimands for its actions ever. This is however unfeasible for several reasons:
- There is always more than just one enemy for any nation.
- The speed of conquering is severely limited.
- The frontlines are so wide that you will be invaded somewhere on your own turf.
- You will never fully defeat even the one enemy you pick to fight (assuming it's another major house and not... like... the Isle of Skye). They will take revenge.
This is neither true in BT nor IRL, partially for different reasons.
You need to keep in mind, the Lyran Commonwealth doesn't invade the Draconis Combine just to be able to doodle a blue circle around a former Combine world on their starmap. They want something and while I'm unsure what it is in the actual lore, I doubt it's resources (those float around all stellar systems for free). What you want is:
accessible resources (ie: already built strip mines and processing plants), general infrastructure, production capabilities, labour force and access to farmland (which would be one of the rarest commodities in all of this). And you want a stageing area for your army to defend your frontlines.
None of this can be used if you just nuke the shit out of a planet and turn it into a wasteland.
Additionally, nuking the shit out of a planet means that
any other nation will do the same to you. This is something that nations (irl and in BT) learned the hard way. IRL, it was both world wars that taught us that all-out-war with ever improving weapon systems do horrible things to whatever place you try to take over. In BT, it was the Tintavel Massacre that marked the turning point. Tintavel was a massive war of attrition fought with nuclear warheads and despite not being the first time this happened, it marked a point where people realized that if this shit continues, every planet will be a useless wasteland by the time they are done with their slapfights.
Even from a purely strategic point of view (and ignoring all aspects of humanitarianism) this is incredibly bad:
It means you lose a shitton of military assets and you get nothing valuable in return. It's literally the trenches of WW1, it's a giant meatgrinder that consumes men and material and all you gain is a few yards of frontlines that have no use whatsoever outside of being bombed by an enemy that just dug in a few yards further down the frontlines - but now on a stellar scale.
Up until Tintavel, the IS didn't give much of a shit, there's a scene in Futurama where Bender litters, Leela tells him to take better care of the planet and he replies "why should I? It's not the only one we got" and that kinda sums up the IS and their attitude, too. Then Tintavel happened and everyone realized that with an attitude like that, it was a race to the bottom.
The Ares Convention ironically did something that you also allude to in your post:
Rules for warfare legitimized it as a political tool to realize one's ambition. When you know that your enemy won't go all in with nuclear hellfire, you can attack him at your leisure. War became far less destructive and maybe smaller in scale, but also something more common.
With the break-down of the Star League and the first Succession War, focus of war changed from "I want something" to "I want something and if I can't have it, I'll smash it, so no one can use it against me" until the IS relearned their lesson (and incidently, grew so weak, they would not have been able to continue even if they wanted).
BT uses Realpolitik masterfully as a foundation for their setting. When BPL Tex says that BT is GoT before GoT even was a thing, he's absolutely right.
The problem is actually getting there to do it. Four Succession Wars popped up and grounded to a halt before anyone could get to the other's capital, and even when they threatened to get there, there was more than enough advance warning to move the capital. The Clans did it... with the element of surprise, massive technological advantage, armies that weren't depleted from multiple Succession Wars, and ComStar backing them up.
The scale and travel mechanics of BattleTech simply don't allow for that sort of strategy to work. You either try to do a deep push and get engulfed and destroyed by the enemy, or you try to push a front and lose steam. Travel is also very slow (it can take weeks to go from a jump point to a planet), and WarShips were only just beginning to be deployed again when the Jihad popped up.
The concept of MAD only worked on earth cause the duration between the start of an all-out nuclear war and the end of an all-out nuclear war would be like an hour or so.
Both the USSR and the USA were capable of reacting to a nuclear strike within minutes of it launching towards them and most nukes would be on their way in retaliation before the first barrage even hit. All thanks to sub-orbital ICBMs and bases around the globe to deliver bombs to every fucker that gets twitchy.
By comparison, travel times in BT are so ridiculously long, it's like a soviet nuclear first strike against the USA would use a horse cart to transport the nuke. You wouldn't even have an element of surprise, cause FTL communications are still a thing in BT (and one of the reasons why war became less "scorched earth" I presume).
In short, it seems weird that war -something utterly uncivilized- would use rules, you'd think it would always be as savage as possible to overthrow your enemy, and that is not that far off the mark. But when you look at Earth's history, you'll quickly see that war had rules, even before nucelar war was a thing. The Geneva Conventions started out as a pact to help drowning soldiers after a sea-battle, just cause everyone agreed letting thousands of people die a horrible death. Then people agreed to treat POWs with dignity. In WW1, fighter pilots would not shoot an enemy if he ran out of ammo and became defenseless. In WW2 German pilots would escort heavily damaged allied bombers out of German airspace on occasion. German U-Boat commanders radio'd in the position of British sailors marooned after their ship sank (and only stopped after those utterly disgusting assholes in the British high command used that to sink the U-Boats via airplanes in retaliation). Certain types of weapons aren't used cause they are deemed "unfair" and "cruel".
In that regard, the rules of war in BT are absolutely sensible.