The Failed Sphere.
"For the Tau Empire, the Third Sphere of Expansion was projected to take our species beyond the Great Void. A Great Leap for the Empire, in the name of the Greater Good, like the two that preceded it.
In the First Sphere, we expanded beyond our home system, meeting the threat represented by the barbaric Orks…and we overcame them.
In the Second, we fought the Gue'la of the "True Human Empire of Thebes", which had forced its expansion in our region. We repelled their armies, and learned of the Imperium from which they had escaped. The prisoners we captured spoke of a vast empire, with numberless armies that once were theirs to command, but had cast them aside in the name of a heretical cult which they called the Heresy of Fire.
When the Ethereal Supreme, Aun'va, launched the Third Sphere, aimed at conquering the area of space located beyond the Great Void (Tau name for what the Gue'la call the Damocles Gulf) we assumed it would have been like the others. An astounding success for the Greater Good.
At first, the only opposition we met was from independent Gue'la worlds and Ork petty kingdoms. We conquered them, their forces no match for the Fire Caste and the diplomatic abilities of the Water Caste.
But then, when we ventured beyond the Gulf, we found the most powerful enemy we had ever faced up until then.
We found the Imperium of Man the Theban gue'la had warned us about.
And from the east, a never-seen before threat came, hungering for our flesh.
And for the first time, the Tau knew defeat.
-From The Failed Sphere, written by Aun'El T'olku Naseon. Text issued as required reading for all subjects of the Tau Empire in 997.M41.