I am currently playing Dawn of War - Soulstorm and I am wondering why Space Marines of the Blood Ravens Chapter fight The 252nd Conservator Regiment of the Imperial Guard, since both Space Marines and Imperial Guard are Imperial troops. Is it just for the game's sake and the lack of diplomacy in it, or can Space Marines fight other Imperial troops? Did the fight between two imperial forces happen in 40k lore anywhere?
That's really only for the sake of the game.
Imperial troops kill each other all the time, for a variety of reasons. But you never really come across protracted wars between two Imperial forces (who aren't being coerced or tricked in some way), like we see in the Kronus campaign.
The Inquisition for example, will wipe out entire planets of Imperial citizens if they deem it necessary. If enough of the population has fallen to Chaos, or even if only a tiny percentage do and the rest don't rise up against them quickly enough... It really doesn't take much, the Inquisition are pretty trigger happy.
But I don't think there are any examples of two loyal, uncorrupted Imperial forces engaging a long drawn out war over territory.
Oh yes... many times!
In that game in particular everybody has their own agenda so it a free-for-all.
Dark Angels eradicate any allied troops that see their hunt for the fallen.
Blood Angels can kill allies if their Black Company goes berserk.
Deathwatch & Grey Knights & Sisters of Battle kill off allies if there is a risk of Chaos or Xenos taint.
The history of the Imperium is full of stuff like this.
Then you have Rivalry inside the Imperial guard regiments. Best place to read about is in Dan Abnett's Gaunt's Ghosts books. There you can see that different regiments fight against each other. FULL on WAR between Tanith First&Only and Volpone Bluebloods
Yes it happens frequently. Imperial forces may rebel either of their own volition or because they have become corrupted by Chaos or Genestealer cults or simply becasue their commanders decide to rebel. Indeed during the Horus Heresy elements of almost all types of imperial forces defected to Horus including Adeptus Mechanuicus, Titan Legions, Imperial Guard and local forces.
Also with long distance communications being patchy at best and the overall culture of secrecy and absolute obedience in the Imperium accidental conflicts between nominally loyal forces are not uncommon.
Equally Imperial commanders, especially the inquisition may decide to exterminate allied forces for reasons of expediency, for example any forces other than elite units like Astartes are likely to be eradicated after coming to contact with sensitive information such as the existence of Chaos.
Indeed in one notabel case, in the aftermath of the 1st Armageddon War the Space Wolves came to blows with the Inquisition and Grey Knights during the 'Month of Shame' when they attempted to prevent purging of Imperial Guard and local forces who had fought against Chaos.