We don't really know (at least as far as im aware) how well known a threat the Tau are in the imperium, do the High Lords of Terra even know of the Tau's existence?

Sure we know that they had the Damocles Gulf Crusade called against them but as was mentioned before this was called by a fairly local Cardinal who could have called the crusade without much need for consent from higher ups so who knows how much the Ecclesiarch and any of the other high lords would have known. Especially when considering the vast bureaucracy of the imperium and how long information can take to reach terra from that distance.

At the moment the Tau only present a very localized threat so I would suspect it is something they would leave in the hands of local authorities to deal with such as the Ultra Marines, local inquisitors, governors or priests. After all the imperium has a lot of bigger fish to deal with so I doubt the Tau get much thought from the makers of overall policy at all really.

Nevar:
The concept of the Tau Empire is that they advanced incredibly rapidly beyond any expectations. Yes, the Imperium did undergo a period of advancement, but to say that it even rivalled theirs goes completely against the basic idea of the Tau.

They advance rapidly, but they are just now getting to human equivalent tech levels (comparable weapons strengths and military capabilities).

Point is, Humanity stopped advancing based on a disaster in their history which the Tau have not yet had. Point is, the humans could even have the last laugh when all the gundrones rise up and revolt against their masters. *shrug*

And just imagine what would become of the Tau if that technology advances even more! By the time the Tau has been around as long as the Imperium (if they don't get destroyed ), they will be an overwelming force of sniper soldiers. Humanity has to understand the conflict of intrest between them and the Tau. they cannot constructively defeat the Tau without losing a major ammout of their own. That will only get worse as the Tau progress.

What fun for us

Exactly, humanity got really lucky and just about survived it's apocalyptic singularity event (which I like to think had more to do with the consequences of uploading human minds like they do with animals for titans, or what happened to the necrons, than the slightly more cliched AI rebellion). The Imperium now, in all it's seemingly pointless harshness and peculiar anachronisms, is set up to last whilst avoiding another apocalyptic technological singularity, an Eldar style Fall into decadence or just fading away due to lack of purpose and falling numbers.

It's a system that exists purely to perpetuate itself, fitting nicely with the various 1984 nods and themes. The Tau also look like they are aware of the issues, to some extent at least, given their reluctance to tackle the short lifespan of an individual Tau despite all the rest of their genetic tinkering and selective breeding alongside intentionally limiting certain technologies (like battlesuits). The Ethereals seem to be trying to skirt around the edges of the singularity.

The Dark Age of Technology was (I think) caused by the Birth of Slaanesh isolating the human colonies (I'm not certain this is the case, but the background says it does if I remember correctly. This does raise the interesting plot hole that if the Birth of Slaanesh finished off both the human and to a greater extent Eldar races, then both races reached their height at the same time and were codominant together). The point is, only a Psychic event or Tyrannid-esque invasion could threaten an empire spanning the entire galaxy. The Tau are immune to the former and coping reasonably well with the latter. The revolt by the Men of Iron in humanity's cause is unknown, but most probably chaos related. If you look at the falls of the great empires,

Eldar - Arrogance, Chaos
Man - Chaos, (Dogma?)
Necrons - Greed

I can't see Tau falling so quickly if they stay true to the Greater Good (Rebellion by integrated species, eg Kroot?!)

Just my (extremely optimistic) thoughts.

If a fall were to come about I agree it'd probably be a revolt against Etherals (look at the farsight enclave). Perhaps they do emit a pheromone and perhaps the tau are becoming immune to it or other races are too. Time will tell though.

The enclave are between a rock and a hard place though. The imperial blockade and the Tau fire caste. I'm not so sure that they will last very long. The chaos have their own place and are very much unchallenged there.

The Greater Good is not really about Good however. It is about submission of the individual to the order of the society.

The Tau are essentially the anti-Chaos, or in other words, Order.

Exactly. Provided none of them start getting ideas of individual power and dominance, the Empire should be fine. This is naturally enabled due to the short life times of the Tau.

(As a by-the-way, I find the similarities between the Tau and Necrontyr startling. Both have short life spans, are physically weak so rely heavily on technology, and are immune to chaos. I find it interesting that there is the possibility that if the Necrons hadn't succumbed to the Immortality of the C'Tan, and instead tried to make the best of it, they could have been Tau.)

Age of strife is when humanity's colonies are isolated from each other by the warp storms caused by the developing Slaanesh, ending with the Fall of the Eldar and the birth of Slaanesh. The warp storms cutting off contact ends the Dark Age of Technology (which in another setting would be the golden age, but since you can't trust technology in 40K, it's a dark and godless time).

But surely this was seen as a golden age at the time, just now it is viewed as bad because the current government dislike technology? It certainly seems to be when humanity is at the peak of its power, just heavily subsidised with technology, so you still get the two races having simultaneous periods of galactic domination...

Reminds me of a campaign my local gaming club ran. WH30k...as the only Tau player, I modelled up a single "Stone Age" fire warrior armed with a primative bow. That was my force! (no, I didn't actually play...just sat it out.) But seriously, The rate of technological development seen in the Tau means in a few decades, the Tau will have a strong tech advantage.

Remember though, we have plenty of real-world examples where a high tech army does not always win, especially if out numbered and out indoctrinated. (Vietnam vs France, Vietnam vs USA, Third Reich vs USSR, and USSR in Afghanistan all come to mind)

The Imperium has the capacity to simply swamp a force like the Tau if it becomes sufficiently motivated. A clever Tau leader would be very careful to not give them the motivation to do so.

Hownow: The difference between reality of history and the Tau is that they are an alien race, and the Imperium is locked in many simultaneous campains to focus on any particular front, giving the Tau some room to expand technological advantages.

In all four of those examples, it was not soldiers that outdid the technology, but the geography (forests, forests, Winter x hell, mountains). If it really gets that tricky for Tau, a railcannon from a starship should do the trick (take THAT mountain/forest/glacier)!

There was a lot more to those losses than purely geographic factors, but getting into that is somewhat off topic.

The Tau have advantages on the Tactical Level when forces of approximately the same echelon meet in battle. A Tau Cadre and IG Regiment and a SM Company are about the same in capability, though leverage their abilities in different ways (Technology, Numbers, Supermen).

In terms of Grand Strategy, the Imperium hols all the trump cards however. They are far more numerous, they are better prepared for an interstellar war on massive scale (having fought them non-stop for about 10 000 years now) and have much faster starships. If the Imperium were to decide to crush the Tau and put the appropriate resources into it, the Tau would fall, probably without doing much damage directly to the Imperium.

However, the Imperium can't go to war with the Tau on that scale because they are already at war with Chaos, Dark Eldar, Orks and Tyranids. Diverting the resources necessary to crush the Tau from these conflicts will mean that a far more dangerous opponent will push deeper into the Imperium. The Tau survive because of this more than their technical or tactical acumen. As long as the Ethereals realize that it is for the greater good to expand diplomatically first the Tau should avoid the threat of extermination which hangs over them like the sword of Damocles.

There are also Imperium commanders who understand this and have put the Tau at the bottom of the list of urgent priorities.

The best bet for the Imperium to survive is to do the one thing they can't do - withdraw. Much like Rome found itself unable to defend all her territories and had to leave Britain to the Saxons, the Imperium needs to pull back and consolidate into a smaller, more easily defended territory. Once they have stabilized the front and given themselves a chance to rebuild their numbers instead of just barely sustaining them, then will they be able to burst forth in another Great Crusade.

I'm sure it was (though it lasted long enough that it probably just became the new 'now', and for most of the population to get used to high level tech being just something that was there rather than something to strive for). But it definitely ended before the birth of Slaanesh, which is what paved the way for the Age of Imperium after the millennia long Age of Strife

My point is that the Eldar empire was ALSO ended by the birth of Slaanesh. Given that the mechanicy people (forget their imperial name) get most of their 'new' stuff by finding old Standard Construction Templates, this was incredulously more successful than the current Imperium. I think as well the Emperor spent most of his time humie-bashing the empires of humanity that were separated from Earth but spread over most of the galaxy. So you have two dominant species sharing the galaxy. Given that both appear to think they rule the entire galaxy by some sort of divine right, I don't see how they got along before Slaanesh.

I just noticed this on another thread, but thought that it would be useful to post in this topic.

The Tyranids have been slowly emerging from the Eastern Fringe, and have been consuming life wherever they went (well, until they're latest codex was nerfed, but thats irrelevant). The Tau Empire lies directly in the path of the Tyranids in the Southern sector of the Eastern Fringe. Immediately, one would think that the Imperium could use the alien Empire as a buffer between them and the Tyranids, easing the load placed upon them. What was mentioned in this other thread was that if the Tau fail to defend against the Tyranids and are ultimately consumed, then the Tyranid Hive Fleets will be even more powerful than before having recently consumed an entire Empire.
Now, if the Imperium were to call truce with the Tau, then they could hold the Tyranids at the border of Tau space. Not only would this allow for better strength forces to repel the aliens, but it would also mean that any worlds lost in this defence would be lost to an alien race, and not humans.

I just thought that this would be an interesting mention.

That makes sense. A form of truce can be made to assist against tyranids only, slightly off their "hate all xeno" standards, but in this case, i would call it "i hate them more". Maybe a universal peace will never come, but a common ground of survival may allow an alliance to fight the tyranids side by side, yet still hate each other on every other front. Then, the Imperium would still blockade the Tau from gaining more territory, and keep the tyranid at bay.