I'm slightly confused, how come creating an alliance with Xeno-life is such a crime in the Imperium, yet in the space marine codex, they have this:



Even if they had no choice but to ally, the Ultramarines could've destroyed the Tau with the Necrons on Malbede.

Why didn't they?

Because it's Imperial propaganda to portray their Space Marines in a light any other than the reality that they are cold, evil remnants of a dying empire.

((AKA, codex writers might've realized that having the SM ally with Tau, then backstab them, might possibly be evil and better belong in the Chaos or Dark Eldar codex. And the above was 'In-Character' by the way. ))

Well i've heard that the IG did that to the Eldar once(but I can't remember where it is said ). so it's not completely uncalled for to the Imperium to "backstab" a xeno that may or may not have just saved your life.

The Marines fought side by side with the tau against the implacable skeleton death machines. Not only does going into battle with others build bonds in ways normal interaction cannot, the marine commander did the honorable thing and allowed his foe to retreat. He still blasted the planet to smitherines, but allowed his previous allies a chance to withdraw. If the Tau had stayed, he would have still blown them to high hell.

Think of it as warrior's honor. He wouldn't bat an eye lash if he had to bombard a Tau civilian center to rubble during a war, but he isn't going to betray former allies right out either by burying them under the inquisition's wrath without so much as a warning as thank you. Sort of a begrudging respect sort of deal. Likely the Inquisition would have pressed him to just incinerate all the xenos as that is pretty much Imperial policy. But the Chapter Master of Ultramar sorta has enough weight to do what he wants to a degree.

I agree entirely with Nevar who sniped my idea. To add...
Space marines are human. Super human, yes. But still human. They are as capable of being cold hearted evil killing machines as anyone, but they also have honour. I feel many space marine chapters pride themselves very greatly on their sense of honour, often more greatly than their "everything that is not us must die!" mantra. I wouldn't even think its propaganda, more the idea that the marine's themselves felt that they were too good to wipe out their temporary allies in a dirty double cross. The next time they met I'm sure the marine's would have no worries about destroying the Tau and wouldn't give it a second thought.

Well you have the old cliché, “the enemy of my enemy is my friend” basically you can negotiate with the Tau Empire, and you can’t really do that with the Necrons. As for how different SM chapters interact with the aliens of the universe is going to differ from chapter to chapter. Some chapters will work with, and or leave alone Eldar, others will hunt them down mercilessly regardless if the Eldar are coming to their aid or not. In this case, the Ultramarines knew the planet they were fighting over was doomed, and because of the aid given to them by the Tau with the temporary cease fire, Calgar allowed them to evacuate the planet before issuing exterminatus. Given that Ultramar sits on the eastern side of the empire and has a lot of interaction with the Tau Empire will not surprise me to see Ultramarines and Tau fighting side by side against a more terrible foe. Even the Imperial Guard has been known to use Kroot Mercenaries from time to time. It comes down to fluff mainly, and who is dealing with whom.

Thurgen

Also, the Empire might take extra exception to any treachery and this would contradict the whole point of the space marines to DEFEND humanity, and not aggravate an empire who could strike back (admittedly probably not against Ultramar ). The whole point of them is to pen the Tau back after the failure of the Damocles crusade to destroy them outright. Also, the Tau are, as others have said, civilised enough to ally against future barbarian Orks, Tyrannids, Necrons, etc and if they knew they would be blown into seven hundred small, blackened, irregularly sized pieces afterwards they might not do so again.

I think everyone has hit the nail on the head. The Ultramarines pride themselves on being honorable warriors, so it would likely have been distasteful to turn on former allies. However, there are plenty of other Chapters who would have refused to ally with the Tau at all and would gladly burn the planet along with them if it came to that.

The Tau Empire represents a very different type of threat though. They don't simply want to destroy you, like so many other xeno's, but instead they want you to join them. They spread something very dangerous to the Imperium: an idea. So it's likely that the Imperium fears contact with them for the simple reason that they may plant the seed of doubt in the minds of some Guardsmen. Marines on the other hand are heavily indoctrinated and would not fear that any of their number would join the Tau, but they might view it as an admission of weakness to rely on them. I think the term is "monodominant," but it is the viewpoint among some Imperials that humans MUST be the only form of intelligent life that should exist in the universe and all others must be destroyed. Other Chapters would likely have a more pragmatic view, acknowledging the futility of such a crusade.

When it comes right down to it, the Space Marines know the score. There's a hierarchy of evil in the galaxy, and while it's they're policy to hate everything equally, the reality just isn't that simple. Space Marines, possibly more than any other aspect of humanity, have had the most exposure to Xeno's and their varied agendas. A conflict with the Tau represents the loss of territory - loss of life only occurs during instances of resistance - they're a known quantity with a well understood agenda. Tau occupancy just doesn't hold a candle to the nightmarish actions of the Necrons, Tyranids, or Chaos factions. Imperium propaganda is designed to hold untold trillions in line and loyal, and the Space Marines act on it when needed to - but their are many instances of the wisest of Space Marine leaders reaching out for assistance from fair-weather friends. Take Dante's defense of Baal against the encroaching Tyranids as example. He understands that mutual survival is more important than Imperium logic.

Ok, I can understand why they didn't now but 1 thing still eludes me...

If the superstition to hate and distrust xeno is high, then how could the SM trust the Tau from not blowing them up? (I know that the Tau wouldn't but the SM didn't right)......

Well if the Ultramarines are issuing Exterminatus, you can assume an Inquisitoral fleet arrived, or enough imperial ships to lay waste to an entire planet. Most likely these 'reinforcements' dwarfed the Tau forces by a very large margin. Enough to ensure the safety of human forces even in the off chance the Tau decided to strike at them.

The Ultramarines have the cabability to preform an exterminatus without the aid of the Inquisition. (referance Novel Nightbringer) But again, Calgar has an entire area of space to run that bumps into the Tau Empire. With Tomb worlds waking up, Tyranids at the door step, Orks everywhere, and Eldar pirates it would behoove him to try and keep a level of honor while dealing with the Tau because who knows when he might need to ask for their help. The south eastern part of the empire is a long way from Terra, and the high lords strict dogma.


Thurgen

He may be a long way from home now, but he will eventually have to go back when needed and then problems can arise in that scenario. Also, if they were to perform exterminatus alone, then my concern still applies because the Tau could have position themselves to kill major chains in command if the SM turned their back long enough.

Such an act of betrayal by the Tau in that situation could result in another devastating crusade. The assassination of Calagar would have brought about an extreme response from the realm of Ultramar. That response would cost the Tau empire dearly, and with new threats encroaching on the Tau empire do they really need to provoke full blown war with Ultramar. It would be one thing for him to die in honorable battle, that can be accepted as what comes with being a space marine, but to be betrayed when you are giving aid is another. However I do not believe it is in the Tau's best interest to grow the hatred between the races. Ultimately the Tau want to absorb the Empire into itself, and I believe the Tau can ill afford a full blown war with the Empire. It is one thing to take on the Empire in localized conflicts for small grafts of territory, and another to invite full open warfare.


Thurgen

In the Space Marine codex the Zeist Campaign refers to the Tau as fighting along side the Ultramarines against Hive Fleet Kraken, so one can assume that it is neither the first, nor the last time that the Ultramarines will join forces with the Tau.

Something to think about in regards to the Empire, Ultramar, and the 'nids is the simple fact that we are a bit of a speed bump. We've displayed proven success against the 'nids in more than a single theater. Now I ask you all to think like a large scale political leader. The fact of the matter is that The Tau cannot afford a full scale war against the Imperium or even just Ultramar. They do not have the resources necessary to fight the prolonged war that could come of betraying the Ultramarines. Remember, there are many myriad foes in the galaxy. We have enough fronts closing in on us without creating another enemy on the offensive.

On the flip side, Ultramar can well utilize the Tau. The Tau empire blocks off an avenue of approach for the 'nids. It eliminates chaos forces with little to no risk of corruption. And it must always be remembered that in war the rule of thumb is "the more you use, the fewer you lose". This simple truth can be mitigated somewhat by training and equipment. Ultimately it simply means the bigger your hammer the less damage it takes from the nail. Think of it like hitting a light pole in a prius. The pole will fair better than the car. Then run the same scenario in an Abrams going 60 mph, roughly 120 kph for the rest of the world. The greater application of force will eliminate the foe quicker removing the enemy's ability to inflict casualties. The best way to clear a building is to reduce it to rubble after all.

Point being, if the Astartes (the smart ones at least) can bring more rifles to their side of the conflict they'll fare much better. As long as the Tau and Ultramarines keep in mind their mutual benefit they'll maintain an honorable "truce". Both sides are bred for war. Both sides can put aside past feuds in the face of a common foe. After all if in one conflict I agree to a truce so we may collect our wounded and dead; you'll be more likely to side with me at the threat of mutual destruction from a third party.

While I understand your usage of the tactic, I must point out that the "the more you use, the fewer you lose" saying doesn't really apply anymore, nor has it for a long time. Such a philosophy would be correct in the ancient times and even up to the late eighteen hundreds. However as war has gotten more modern, the importance of precision has become tantamount.

The US Army and Marines held off many more their own number during the Korean War by adjusting their tactics and the massive Chinese army found it's numbers almost a hinderance instead of an asset after their initial ambushes.

My point is, when it comes to the mental state of the Astartes I am not sure such a force would consider that military philosophy. They are the masters of surgical precision strikes after all. Your example would apply more to a Imperial General when sending in the two-hundred and sixty seventh wave of guardsmen. However nearly ever major fluff that has cooperation between the Tau and the Empire, it is the Astartes that are head of such brief alliances.

I would counter that a warrior might love his sword and hate the pike, but sometimes you need the pike to face down a charging horseman. The Astartes might prefer human assets and their own personal abilities, but when the beast is bearing down on you and there is an effective and controllable tool or asset to aid you in your task... it's only common sense to employ it. I doubt they considered the galactic stage when they allowed the Tau to withdraw. In my opinion it was more an act of personal warrior's honor.

Under the right conditions a small, well equipped, well trained force is far superior. In fact most of the time training and equipment will outweigh numbers. Over all the tactic remains exactly the same. It isn't just numbers. It's force. The whole point of a surgical strike is the perfection of this. It allows a small force to be overwhelming. It's overall combat capability, not numerical advantage. Ancient example: the use of knights as heavy cavalry with barded destriers. There were only two things really effective against them. Your aforementioned pikes, and terrain that favors infantry. The cavalry would be fewer in number but a far superior concentration of martial force; being better armed, armored, and trained. A more modern example would be the battle for the Golan heights. The Israelis were far outnumbered but had superior equipment overall. They could engage at a far greater range than the soviet built armor they defended against. And the only reason they took the casualties they did is due to being far outnumbered. It's force not numbers in the end though. A numerical advantage is always handy but not the point. Not to mention one person can only ever be in one place. You try doing twelve hour guard shifts where the most interesting thing is an Afghan chasing down a donkey that got loose.

I wouldn't over-think the situation much to be honest. It really just comes down to individual instances of mutual survival. Sometimes an individual who doesn't entirely subscribe the the Imperium's propaganda finds themselves in a position to make a concession and they do. But unless humanity undergoes a 180 degree shift in policy, the Tau will eventually have their turn on the chopping block - assuming everyone lives long enough to make it there.

As far as retaliation can go with the Astartes, they don't know much of the Tau's inner realm's power, which can give the Tau an advantage in homebound conflict. Of what I have read in fluff, the Imperium does not know the internal workings and numbers the Tau represent. And the Tau Empire has fewer fronts to protect than the Imperium so the Tau can better cluster their forces, given that the Tau know they're coming. The Imperium has a surplus of Imperial forces but they are far spread across the galaxy against powerful foes. The roman empire represents the Imperium in a sense: A seemingly unstopable force spread thin and their foes utlized their error. The cadian gate, the ryza system, the Tau's third phase expansion, and more consistently split the imperial force.

The Tau alone would be crushed under the might of the Imperium, but they are not the only threat. They cannot divert much forces to the Tau and favor the outcome that much. And the Damocles showed that the Imperium's force was severly splintered by the Fire Caste and its allies. I don't think Ultramar can fully destroy the Tau under a new command after Calgar if such an event occured.
Depending on who secceded Calgar.