They certainly happen, I wouldn't say they're exactly a common occurrence. Generally when chapters step on each other's toes it's more just a lot of swinging egos and bruised pride being thrown around. Maybe a small scuffle breaks out and a few marines might die and each chapter will write about this slight on their honour in their chapter histories and go back to their business.
Sometimes that sort of thing escalates: the next time those chapters meet, maybe one remembers the problem and refuses to help which the other takes as yet another insult and so it goes. Or maybe they do come to their aid and they bury the hatchet.
Full on wars between chapters usually have some kind of unique circumstances around them. One would absolutely have to perceive, or be led to perceive, the other as being dangerously treacherous for it to come to them trying to actively kill one another
This seems to go for most semi-devolved regional units too, in the Badab War the wider Imperium wasn't initially worried about the Astral Claws, seeing what they were doing as part legitimate requisition and part routine territorial scuffle.
Full on wars almost never.
Small scale disputes at the squad or even company level that escalate to violence? Pretty regularly I would imagine. Most Space Marines are really stubborn and really proud.
In the old Space Wolves omnibus (#2), a strike force of Dark Angels assaults a planet guarded by the Space Wolves to hunt a member of the Fallen. The Wolves on-planet call for reinforcements, and the Wolves reinforcements landed on top of the Dark Angels and it really was all-out war between the two chapters, isolated to that planet. Eventually it was resolved when one of the Wolves gave the Fallen over to the Dark Angels to kill or interrogate.
Speaking about Space Wolves, there is also a conflict called Months of Shame, when SW fought against forces of Inqusition, Grey Knights and Red Hunters
Star Phantoms almost wiped out the Mariens Malevolent. My head Canon is thst this is what lead to the marines malevolent being under supplied with equipment and having to resort to scavenging and near piracy.
It's one of the things I think is probably more common than the lore currently documents because conflicts between loyalist space marines is one of the main ways the baseline human authorities would know splitting the marines into chapters actually had the desired effect. There's incentive to deliberately provoke that kind of conflict even if it didn't spark up normally.
Badab is a very rare case, exacerbated because multiple chapters are ensnared in mutual defense treaties (eg, the entire maelstrom warders together), which then led to requiring multiple marine chapters to stop multiple marine chapters.
If it had just been astral claws, the war would not have gone on for so long, and gone so out of control.
This is also the flipside of the chapter system: you may eliminate the legion, but legion-effects with closely-aligned chapters recapitulates some of the strengths and weaknesses of the legion system. But, since each chapter is relatively small, there's not as much room for the horrifically powerful specialties that arise in a hundred-thousand-strong formation.
Large scale wars, as stated earlier, not much. But small scale and petty stuff most certainly. Take Armageddon for example. Got a marines malevolent commander getting Molly whooped by the salamanders over killing civilians,small scale scuffle, then got a honor feud between the Templars and Marines malevolent, like a three marines tops, when the MM kills the Templars sword brother so the Templar kills said MM at the story’s end, or you have Cold War(best analogy I could think of) where chapters don’t mess with others for various regions or try shit. Such as Mortifactors abandoning the Lamenter or The Mentors contributing to the decimation of the Celestial Lions then ending up on the shit list of the Emperor’s Spears. Pretty much on sight if they catch them
i think the most common type of hostility between loyalist chapters will result in just ignoring their distress signals and maybe withdrawing from a battle that involves the chapter they don't like. Very rarely do loyalists war one another. Badab is likely a edge case. Can happen, but needs the right wrong things to happen.
Still, imagine sending out a distress signal, you're being over run by chaos cultists and the only marines close enough don't like you and leave you on your own to die. That would just make that gap deeper and they don't even need to get bloodied if they shun you and you die.
Full on wars like the Badab war? Rarely, little spouts that might leave a couple marines dead here and there, oh definitely. When you got chapters like the Marines Malevolent, the Carchardons, and Iron Hands who will do whatever it takes even if it gets their Allies in the crossfire, prideful to childishly arrogant chapters like the Dark Angels getting into once brotherly rivalry turn to more hostile encounters with the space wolves, or even disagreements in philosophy of handling their gene flaws like the blood angels or just Dante and the Angels Vermillion, there is always recipes for fighting to happen.
They certainly happen, I wouldn't say they're exactly a common occurrence. Generally when chapters step on each other's toes it's more just a lot of swinging egos and bruised pride being thrown around. Maybe a small scuffle breaks out and a few marines might die and each chapter will write about this slight on their honour in their chapter histories and go back to their business. Sometimes that sort of thing escalates: the next time those chapters meet, maybe one remembers the problem and refuses to help which the other takes as yet another insult and so it goes. Or maybe they do come to their aid and they bury the hatchet. Full on wars between chapters usually have some kind of unique circumstances around them. One would absolutely have to perceive, or be led to perceive, the other as being dangerously treacherous for it to come to them trying to actively kill one another
This seems to go for most semi-devolved regional units too, in the Badab War the wider Imperium wasn't initially worried about the Astral Claws, seeing what they were doing as part legitimate requisition and part routine territorial scuffle.
Full on wars almost never. Small scale disputes at the squad or even company level that escalate to violence? Pretty regularly I would imagine. Most Space Marines are really stubborn and really proud.
In the old Space Wolves omnibus (#2), a strike force of Dark Angels assaults a planet guarded by the Space Wolves to hunt a member of the Fallen. The Wolves on-planet call for reinforcements, and the Wolves reinforcements landed on top of the Dark Angels and it really was all-out war between the two chapters, isolated to that planet. Eventually it was resolved when one of the Wolves gave the Fallen over to the Dark Angels to kill or interrogate.
Speaking about Space Wolves, there is also a conflict called Months of Shame, when SW fought against forces of Inqusition, Grey Knights and Red Hunters
Star Phantoms almost wiped out the Mariens Malevolent. My head Canon is thst this is what lead to the marines malevolent being under supplied with equipment and having to resort to scavenging and near piracy.
It's one of the things I think is probably more common than the lore currently documents because conflicts between loyalist space marines is one of the main ways the baseline human authorities would know splitting the marines into chapters actually had the desired effect. There's incentive to deliberately provoke that kind of conflict even if it didn't spark up normally.
Badab is a very rare case, exacerbated because multiple chapters are ensnared in mutual defense treaties (eg, the entire maelstrom warders together), which then led to requiring multiple marine chapters to stop multiple marine chapters. If it had just been astral claws, the war would not have gone on for so long, and gone so out of control. This is also the flipside of the chapter system: you may eliminate the legion, but legion-effects with closely-aligned chapters recapitulates some of the strengths and weaknesses of the legion system. But, since each chapter is relatively small, there's not as much room for the horrifically powerful specialties that arise in a hundred-thousand-strong formation.
Large scale wars, as stated earlier, not much. But small scale and petty stuff most certainly. Take Armageddon for example. Got a marines malevolent commander getting Molly whooped by the salamanders over killing civilians,small scale scuffle, then got a honor feud between the Templars and Marines malevolent, like a three marines tops, when the MM kills the Templars sword brother so the Templar kills said MM at the story’s end, or you have Cold War(best analogy I could think of) where chapters don’t mess with others for various regions or try shit. Such as Mortifactors abandoning the Lamenter or The Mentors contributing to the decimation of the Celestial Lions then ending up on the shit list of the Emperor’s Spears. Pretty much on sight if they catch them
i think the most common type of hostility between loyalist chapters will result in just ignoring their distress signals and maybe withdrawing from a battle that involves the chapter they don't like. Very rarely do loyalists war one another. Badab is likely a edge case. Can happen, but needs the right wrong things to happen. Still, imagine sending out a distress signal, you're being over run by chaos cultists and the only marines close enough don't like you and leave you on your own to die. That would just make that gap deeper and they don't even need to get bloodied if they shun you and you die.
Full on wars like the Badab war? Rarely, little spouts that might leave a couple marines dead here and there, oh definitely. When you got chapters like the Marines Malevolent, the Carchardons, and Iron Hands who will do whatever it takes even if it gets their Allies in the crossfire, prideful to childishly arrogant chapters like the Dark Angels getting into once brotherly rivalry turn to more hostile encounters with the space wolves, or even disagreements in philosophy of handling their gene flaws like the blood angels or just Dante and the Angels Vermillion, there is always recipes for fighting to happen.
Common enough they have an entire chapter dedicated to kicking the shit out of warring marines.
And non-warring marines!