But do they, really? Plenty of court administered "justice" is doled out to common folk, sure. But for nobles grabbing each other's territory? I'm not so sure. For example, When House Vandenberg had their entire fief usurped, why did its scion and heir have to go thru the events of the original Mechwarrior video game to undo the land grab, as opposed to just filing a grievance with some Davion court? (granted, video game plots are technically considered apocryphal, but it's the most prominent example that came to mind regardless)
actually that is an example that helps show my point.
the plot of that game goes that Anders Moon is under house vandenburg. during a pirate raid, Duke Vandenburg* is killed and an important artifact required for he succession ceremony is stolen. with Gideon too young and the artifact missing, the local legal authorities declare the succession is to be delayed until the heri, Gideon, becomes 23 years old. Then House Vandenburg's rivals, House McBrin, 'discover' some evidence that implicates House Vandenburg in the raid, with the Vandenburgs consorting with the pirates, only to double cross them, leading the pirates to attack in retaliation, leaving Making House Vandenburg lose prestige and putting the succession into question, since the evidence basically implicated Gideon and his father in Treason against the Fedcom. which led Gideon to flee the world, create his merc unit, and seek out both the artifact and proof that the evidence against his family was falsified. if he doesn't achieve that by the time he turns 23, house McBrin would be given control off the world.
it isn't a case of the rival just showing up with some troops and saying "my world now", but rather the rival hiring mercenaries to conduct a false flag raid as part of a conspiracy to discredit another house, while using the legal system to enact a transfer of power.
*that the control of a minor world in a system was a duke, and not a baron or count is probably a bit of early installment weirdness.
Exactly, and what about if the noble who has a fief next to yours *is* your enemy? "Us against them" doesn't fly very well in (neo-)feudalism.
then you get labelled a traitor, and your holdings and forces crushed by house troops sent to put down your 'rebellion'. time of war or not, an outright attack on a neighboring noble of the same house would be viewed as actions against the state, and responded to as such. because as a noble you are obligated as part of your fealty to defend the realm.. which includes your neighbors lands, because they are part of the realm. attacking your neighbor in open warfare is a blatant detrimental act to the defense and readiness of the realm, and thus the higher nobles would be forced to act to stop the aggressor.
far more likely would be the noble deploying his own forces in such a way to leave his neighbor more vulnerable to attack.. like not placing the troops where they can react to enemies hitting the neighbors worlds, or using influence at court to get the house troops deployed to your worlds in large numbers than your neighbor's, making them more attractive to attack.
not to mention organizing raids or invasions of minor worlds on the enemy side meant to gain glory for yourself (and if you are really clever, you raid enemy worlds close to your rival's worlds, so that the retaliatory raids hit your rival, and not you. )
in both cases the goal is to make your rival appear weaker or more foolhardy, and yourself stronger and more valiant.. impressions that you can then parlay into better standings at court and opportunities for social advancement or acquisition of further power in the various noble intrigues.
in many ways the politics of the inner sphere can be summed up at a Ten Thousand Gambit Pile up.. with every nobleman involved in one or more schemes, all at the expense of the other noblemen, at various different levels.
edit: it is worth noting that had blatant internecine warfare with no legal repercussions been a thing, books like
The Price of Glory would have been over very quick.. why would a rogue precentor and the Duke or Irian have to contrive a scheme involving using other mercenaries pulling an atrocity under a false flag in order to discredit the grey death legion and get them arrested and held.. all so the Duke of Irian could grab the grey death's landhold? they could have just landed troops in overwhelming force and taken it. they had to do the false flag atrocity because they had to ensure that when they went in for the landhold, they had clear legal basis to do so.