In the 6th edition rule book for Warhammer 40K, it states that Tau consider (and vice versa) Space Marines and Eldar to be battle brothers. The only encounter I know the Tau have had with Space Marines is during the Damocles Assault (?) where the Tau helped Space Marines hold off against the Tyranid Hive Fleet. The Space Marines let them leave the planet before calling down an Exterminatus on the planet. This seems more like they were 'Allies of Convenience'.

Is there some Black Library fiction supporting this? Some back story I'm missing?

Note: Imperial Guard != Space Marines

  • Are you looking for specifically Space Marines, as I can only find one brief timeline reference to that, or humanity as a whole, since there are numerous instances of Imperial guard allying with them for the Imperium? Commented Oct 16, 2012 at 12:02
  • Specifically Space Marines, as that's who it says they've allied with.
    – AncientSwordRage
    Commented Oct 16, 2012 at 13:02
  • 2
    While the Eldar, the Marines, and the Tau are different factions with different interests, they all share a common desire to protect their people and their civilizations. While most of the other factions are usually too aggressive or destructive to parley even in desperate situations. Commented Oct 25, 2012 at 18:08
  • To build on Mark Rogers' answer, the SM, Tau, and Eldar do have a concept of honor (as different as those concepts may be), and all three races (when not fallen to Chaos) can be reasoned with, and will only double-cross if they have direct evidence that a double-cross by the other party is in the works. The Tau in particular hold out the hope (as often dashed as it may be) that the other two will someday come to their senses and join the greater good. The other major WH40k factions will always be against you, usually in the short term and always in the long term.
    – EvilSnack
    Commented Oct 1, 2016 at 22:41
  • For the record : they are not " battle brothers" (highest chance of Alliance) but "desperate allies" (lowest chance of alliance). Chance for marines a lying with Tai is same as with Necrons: it's a rare, rare thing.
    – Yasskier
    Commented Oct 5, 2016 at 5:51

7 Answers 7


The Tau also briefly allied with the Ultramarines on Dolumar IV during the events of Fire Warrior (both the game and novel) against Chaos.


THE GREATER GOOD - from the Blacklibrary.com

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THE STORY When the world of Quadravidia comes under attack by the insidious Tau, only one man can defeat the aliens and save the planet in the Emperor’s name: the legendary Hero of the Imperium, Commissar Ciaphas Cain. When the aliens call for a ceasefire, Cain expects the worst, and his fears are answered in the form of the dread menace of the Tyranids. As a hive fleet approaches Quadravidia, Cain must try to forge an alliance between the Imperium and the Tau – but can he truly trust the inscrutable xenos?

ABOUT THE BOOK The ninth book in the popular Commissar Cain series.

January 2013 • ISBN 9781849702874

AUTHOR: Sandy Mitchell

  • Do space marines feature in this?
    – AncientSwordRage
    Commented Oct 25, 2012 at 17:39
  • Having read up on this, no space marines seem to feature in this book.
    – AncientSwordRage
    Commented Jun 17, 2014 at 20:17
  • @AncientSwordRage - The non heretical space marine chapters are part of the imperium. So if the Tau allied with the imperium they were allied with the space marines even if they never were featured in battle together.
    – Chad
    Commented Mar 5, 2018 at 15:30

Allies is probably the wrong word. They have fought a common foe before and they have provided intel to the marines and eldar. All three factions seek to destroy the Tyranids. They have worked together to defeat the tyranids, though I know of no formal alliance that has ever been entered. Its more of a the enemy of my enemy is my friend relationship. It has been stated that the marines do not understand the machinations of either race.

Dawn of War


  • 2
    Did this happen in a book or is this just Heresy and Rumour?
    – AncientSwordRage
    Commented Oct 15, 2012 at 19:43
  • This has happened in books and video games.
    – Chad
    Commented Oct 15, 2012 at 20:06
  • In which books etc?
    – AncientSwordRage
    Commented Oct 15, 2012 at 20:15
  • 2
    The Dawn of war books/game for sure. I remember reading another omnibus but not sure which one. It was one with an inquisitor as the head character where the inquisitor was working with the space marines, and the tau and eldar were there.
    – Chad
    Commented Oct 16, 2012 at 13:43

It's actually the other way around. Tau have never joined the imperium, but some excommunicated groups of space marines, have joined the tau for the greater good, along with guardsmen. So, the best way to make space marines with tau, is to create a custom space marine chapter, who is painted in the colours of the tau.

Or else, just treat them as battlebrothers in mechanics, but allies of convenience, in the "fluff" of your battle, such as in the case where they fought together to repel Tyranids :)

  • 4
    Have you seen this in a book? Good point on the last paragraph though.
    – AncientSwordRage
    Commented Oct 25, 2012 at 15:43
  • What instances of Space Marines have 'joined' the Tau? Rather than just allies of convenience. Do you have any evidence or examples to back that up?
    – TommyBs
    Commented Nov 30, 2015 at 15:59

I know Ciaphas Cain forged a temporary alliance between Tau and the Imperial Guard on Gravalax in "For the Emperor" against a Genestealer Cult and their Traitor Army.

For the Emperor


I believe there was one battle when the Ultramarines and Tau combined forces to fight the Necrons on a Tomb World. The "alliance" won and the Tau were allowed to retreat. Then an exterminatus was used to purge the planet of Necrons.


A large Ultramarine force accompanied by a massive Imperial Navy once suddenly appeared in orbit about a Tau planet, and the Tau readied their navy to defend themselves. The Tau were vastly out-gunned. However to the horror/relief of the Tau the IoM forces weren't there for them. An even larger Hive Fleet revealed itself and the IoM forces went to work.

So in this case the Ultramarines were in the vicinity of Tau and not killing them, but instead killing a foe the Tau also consider an enemy.

  • 2
    What is the source for this? Commented Jul 14, 2017 at 21:02

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