The Tau hardly are a serious threat to the Imperium. Annoying, yes, but they're quite isolated on the Eastern Fringe.
Ultimately, humanity fights itself a hell of a lot more than anyone else. Whether they're actually violating Imperial law (read: heretics) or not is completely besides the point-- there are disgruntled Imperial nobility that send their personal armies to attack each other as much as Inquisitorial forces declare populations doctrinal heretics.
The most common non-human threat to the Imperium is undoubtedly the Ork menace-- humanity has fought the orks for far longer than the Imperium has existed, and will continue to fight the Orks long after the Imperium fades into oblivion... and it will, eventually. Between the Tyranids advancing towards Terra and Abaddon wanting to get his hands on the Emperor and pulling him from the Golden Throne, something will eventually sack the cradle of mankind and the Imperium will fall apart.
The Imperium doesn't distinguish between Eldar and Dark Eldar a lot of the time, because they simply don't know the difference. The Dark Eldar fit much more with the propaganda regarding the Eldar, really, so that makes it even harder for a Guardsman to say, "Hey, these guys don't have all the shiny, gem-encrusted stuff..."
Incidentally, the Guard have, in novels, fought the Dark Eldar. The first of the Ultramarines novels, Nightbringer, has a situation where a Guard column gets involved with the dark 'uns. There are a couple other instances I remember, but I can't remember which specific book they were in.
The Guard, I think, are much more likely to face Dark Eldar/Eldar Pirates than Craftworld Eldar armies, in the end. The Craftworlds don't send out an army unless they absolutely have to, and usually, that involves defending a Maiden World. If the Imperium figures out that the Craftworld Eldar are up to something and are attacking a planet they've laid claim to, the Space Marines usually respond, since the Craftworlds are significantly more dangerous fluff-wise than bands of pointy-eared pirates and slavers. The immediate response forces for slave raids from the Dark Eldar are going to be the Planetary Defence Forces, which are a seperate entity from the Imperial Guard to a greater or lesser extent, although on the table they're identical.
Both the countless PDFs and the Imperial Guard are supplied by the Departmento Munitorium identically for the most part, and both fall under the jurisdiction of the Warmaster (not to be confused with Horus, though it's the same title and position) on Terra. The difference is that PDF forces are permanent garrisons drawn to protect a single world, which the regiment is both raised upon and never leaves. The Imperial Guard regiments are mobile and are redeployed where needed. For example, you can have a Cadian regiment which is a permanent garrison on a world in the Cadian system, such as Kasr Sonnen, and they'd still be described as the Cadian whatever-ith, but administratively, they're PDF forces. The Cadian 8th, on the other hand, which is the famous regiment that appears as the "standard" Imperial Guard army in pictures, is an Imperial Guard regiment and detachments from it are sent to the appropriate warzones where needed.
I knew that.
However, it's still incredibly rare to fight any kind of eldar, and it's even rarer for IG to fight craftworld eldar. And even rarer still for the IG to fight Necrons. Tau are fairly common in the eastern fringe. I'll put it in this order:
1. Heretics/rebels. Essentially humans who don't want to be Imperials anymore
3. Tyranids and genestealer cults.
4. Tau on the eastern fringe. non existant elsewhere
5. Chaos Space Marines and Daemons
6. Eldar/Dark Eldar raiders
7. Craftworld Eldar
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