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?