Here's another thinkpiece on the BTech fiction: to what extent can any of the conflicts portrayed in BattleTech be called a "just war"?
First, let's define what we mean by "just war." I'll take the utilitarian approach: a war is "just" when it is better than the alternatives, i.e. the harm done by war was less than the harm of NOT fighting.
Next, let's deal with the obvious objections, at both ends of the spectrum. On one side: BTech shows us a galaxy repeatedly driven to the brink of destruction by war; no war in the IP is justified. On the other extreme: by focusing on soldiers rather than politicians BTech shows us war as an instrument of policy, neither good nor bad.
However, I'd argue the fiction/fluff does make moral judgements about some of the conflicts, though obviously with multiple writers at work it's not always a consistent message.
Unjust Wars
War of Reunification: The periphery posed no threat to the StarLeague, nor were they particularly oppressive to their own people.
Succession Wars: 1-3 were clearly mere power grabs, involving massive loss of life through the use of WMDs.
What about the fourth war? I'm calling it unjust: yes, Max Liao's regime was oppressive, but it wasn't genocidal. The real objective of the war was to create a corridor between the Suns and Commonwealth, and punish Liao for the attempt on Hanse's life. No real net gain for the people affected: unjust.
Minor conflicts (3039, Canopian-Andurien invasion of the Confederation etc.) are generally unjustified, territorial squabbles rather than attempts to limit harm.
Just Wars
Amaris Crisis: a little hesitant on this one. Yes, Amaris murdered the Cameron family. That's one family. Does revenge really justify the deaths required to reconquer Terra? That said, if you let a tyrant get away with murder, there's nothing to stop him doing it again. In this sense, in order to avoid further casualties as Amaris tried to expand his realm, I'm calling this one justified.
Resistance to the Clan Invasion: Easier to justify, as the war was entirely defensive for the IS. The Clan Invasion is unjust: while the damage they inflicted to civilians was minimal (Turtle Bay notwithstanding) there was no net benefit for the people of the IS. They simply swapped a noble aristocracy for a military one.
Bulldog: More ambivalent on this one. The key to justifying it depends (a) on how oppressive the Jaguars were to conquered populations, and (b) theoretical benefits of halting the Clan Invasion for good. Justification (a) seems weak, but (b) is more tenable, so on balance I call it justified.
Jihad/Resistance to the WoB: Probably the clearest case of a just war in the fiction. WoB are an apocalyptic cult that doesn't hesitate to use WMDs and "reeducate" civilian populations.