I'm not talking about the kingdom-scale wars like England vs France, I mean more like counts or barons of the same kingdom fighting each other for their lands. What would happen in those cases? At what point would the king intervene or would he interven at all? Would they even happen at times of large scale war?
As for the period, was this a less common occurrance as for the later middle ages or was it still a thing during them?
Well keep in mind a baron’s lands were never actually his own lands.
The idea behind feudalism was always that the crown still owns everything. The king just can’t afford to maintain it all by himself. So he leases out sections of it out to the other barons, who lease sections of their section out to knights, and so on. So you really don’t see barons outright attacking each other for their holdings, because that would basically be taking from the king and we can’t have that. In the cases where barons were actually trying to take each other’s lands by force it was almost always during an overall civil war involving the crown itself.
Now if a baron dies without heirs to continue the lease or he does something worthy of punishment to lose his lands, then they are taken up by the crown. The king can then choose to lease them to another baron if he wishes, so you can imagine there was a bit of conspiring going on between the king and barons. If someone disliked a baron or wanted his lands bad enough, they could arrange for his death or frame him for treason, and make an $arrangement with the king to be given the land.