Which factions fight against each other?

So, I had been thinking about all the factions and more specifically (if you read the title you probably know) do they ever have conflicts where both sides are the same.
So, my thoughts on few of those are here:

Space Marines (any chapter):
Would probably say no, other than some tiny fight in some cool book-plot where there is conspiracy and stuff... but with two different chapters, would they ever engage in war against each other? Would they?

Imperial Guard:
I see them more likely than the one above, because there are so many of them out of many different worlds. Maybe at some point two commissars just go crazy and go in a private war, throwing all units at their command on each other. Then at some point it would fall apart but it would be a lot of casualties anyway.
Skipping sisters of battle, know too little of their fluff.

...skipping orks as well. Because
Waaaagh!

Tau:
Probably very little, except I don't know if some kroot ever rebel or something.

CW Eldar:
Very little, since lets face it, they value their lives a lot more than we do   
But I can still imagine two autarchs who declare a duel and send squads fight in non-lethal combat to see who wins, and if somebody dies by mistake, one of the autarchs tells about it to the farseer and blames the other one.

Dark Eldar:
Well. Just about as much as the orks.

Chaos (Marines):
A lot more than regular marines, but they are still pretty unified?



The ones I want most discussion are necrons and tyranids, and Tau as well I suppose, I can't say if they have much conflicts with each other.


Currently I keep this kind of list (not written on paper though):
Lots of internal fighting: Ork, Dark Eldar
Some internal fighting: Chaos Space Marines
Little internal fighting: Imperial Guard
Very little internal fighting: Eldar


Ideas, lore-examples, history lessons, general intuition?
What to move in the list and where to add the missing races?

I think the game model represents a range of warfare, from outright killing, to paintball. A "wound" could represent a dead soldier, but it could also be a "tag", a paintball splotch, or a bit of wool tied round the arm!
A battle between 2 Ultramarine detachments could be a friendly but competitive training exercise, where each marine has one "life", but a 3-wound HQ has 3 "lives".

Eldar esteem their pride and martial prowess very highly; a duel to first blood is certainly in keeping with their ethos.

Tau training exercises would certainly involve electronic "wounds", like in laser tag.

Orks and DE have endless gang rivalry. so would cheerfully fight.

Everyone else represents different political leanings, rivalries and interests.

The only one I have trouble with is Tyranids: even different splinter fleets should be harmonised under The Hive Mind. Internal squabbling is a waste of Biomass!

Chaos does war against eachother all the time. They fight among eachother just as much as orks do, if not more. This is simply because of the chaos gods struggling against eachother. Only reason they sometimes are unified is because of Abaddon or because they wait to backstab eachother for loot and spoils.

Space marine chapters that have bad blood between eachother or after long history do fight among themselves. Many examples that I can't pull out. Rarely all out war, but it has happened.

Necrons would fight against eachother for many reasons. Some Overlords/pharaon wish to reunite the Necrons, some simply wants to extend their own empire and rule over necrons, some are damaged and insane so they just kill everything etc etc. Imagine them being a shattered kingdom and every dynasty/tombworld/ruler is fighting for their own pursuits. And many don't share the same goal of uniting eachother and restoring their rule, unless you're Imothek or the silent king.

Tyranids can be manipulated, there has been some vauge plot and story around it. Examples are a chaos warpsmith in the Sentinels of Terra book. So a reason for tyranids to fight eachother could be that, one of the splinter forces have been infected or something and don't serve the Hivemind. Another reason could be because the Hivemind(GW) feels it need to evolve and bring new stuff to the table to fight new dexes (and earn money), it wants to test its own forces. But pitting them against eachother and analyzing their own weakness etc etc. See which splinter force is stronger and how to improve. And here, they don't do supervised test and stuff. They go all out real and serious. Not like Tau and their electric toys!

edited many times...

SM: There have been alot of SM on SM action, not including the HH (because thats a whole can of worms on its own) we have the Badab Wars and other times where chapters go against other chapters. Its not often ofc but there have been quite a few times small skirmishes have happened.

IG: Guard vs renegade PDF forces??? Add in most Renegade forces dont know they are actually under the command of a "renegade" commander. They are just doing their duty to defend what they are ordered too. Its sad really but many a friendly fire casualty has happened between say, SM and guard, because of not knowing the truth behind the orders.

Orks fight all the time because they are bored

Tau: There is the whole Farsight Enclave thing. Im sure they have come to blows against each other because of that. Other than that the Ethereals keep them in line most of the time.

CWE: actually there have been stuff about them fighting each other alot. Craftworlds disagree all the time. If I remember right there is fluff about Saim Haim full out attacking other CWE for landing on their planets.

DE: see orks for details

CSM: They fight all the time for power and position. They also try to steal other Lord's bands and fleets to increase their size. Infact the only real "unified" CSM leaders out there are Failbbadon and Huron Blackheart. Everyone else just kinda goes around and hopes they dont get beaten by their own "allies"

Daemons: The gods fight among themselves all the time, enough said

Necrons: There is fluff about stuff like the Destoryer virus planets trying to invade other Dynasties. Also they are just the cranky old men of 40k now so Im sure Dynasties have fought each other over messing up their lawns or maybe stealing their morning paper or something stupid like that...

I've no foundation for this in the fluff (and haven't read much about it), but I think of Tyranids as being a bit like an ant colony. Following that model, all parts of a hive fleet operate towards the single goal of the fleet, but that does not necessarily indicate that there is any species based loyalty. In a couple of species of ants the colony will conduct raids on other colonies and take and eat the larvae, for me it would fit with the homogenizing behaviour of the tyranids that if they came across another hive fleet it would be all out open combat, and I could very easily see a large fleet entirely consuming a smaller one in order to convert it's biomass into its own genetic material.

Although it means that rival Tyranid players shouldn't play each other, I disagree: the whole Tyranid race is a gestalt entity. What the Imperium views as Splinter fleets are just the vanguards of differing tendrils of the galaxy-wide infestation.

What few records there are of Tyranids that have broken free of The Hive Mind always portray them as independant creatures, or at most a rare strain of an individual species: Ymgarl Genestealers, The Red Terror, Catachan devils. They do not co-operate with other Tyranids.

I'm lucky that way: my army of Skaven-Counts-As-Tyranids view ALL races as inferior, and thus worth attacking!

I have always thought the Tyranids as the most depressing race... Humans are so lucky they don't know they are coming!
I'm just glad they don't know how to get to the webway, so we are safe!   

I always imagined the Chaos Marines have alot of iternal fighting, seeing as how the different gods like to fight against the other ones alot.

I would add that Tyranids engaging in combat with another is quite a good way for the Hivemind to 'stress-test' it's creations. Also Tyranids seem to engage in a much greater rate of improvement when involved in outright conflict, think of it as another method of forced evolution and either way no biomass is lost as the winning fleet will simply absorb whatever is slain. Even on a small scale any casualties, be they single hormagaunts or full blown hive-ships can be reconsumed and broken down as biomass by one or both of the Hive fleets.