For The Greater Good

The Tau Empire rapidly developed over the next thousand years, both scientifically and in size. Within a short period of time, the Tau developed rocket technology and quickly established colonies on the moons of T'au. Access to space allowed them to develop larger vessels, which in turn allowed the Tau to travel to nearby planets. Air caste stellar cartographers discovered that the Tau homeworld resided in a dense cluster of stars, and a supernova could wipe out their entire race, prompting further expansion. The Tau discovered the existence of the Immaterium and warp technology from an alien space vessel found on one of the moons of T'au, but as their race had no psykers, they were unable to tap into the full potential of warp travel. They were able to develop limited faster-than-light technology by modifying their gravitic propulsion system to allow a starship to 'dive' towards the Immaterium, the resulting expulsion throwing the starship a significant distance. With this new technology, the Tau began to expand into interstellar space, claiming new systems and discovering alien species. The Tau continuously establish new colonies. Unlike other galactic races, including humans, if a new world is already populated they do not exterminate native race, but allow them to continue existing as part of the Tau belief in unity and peace (the Greater Good). The Tau believe in letting others believe what they want (including the worship of the Emperor of Mankind), but must be loyal to the concept of The Greater Good.

Allies:
Total War: Great Britain
The Kingdom of Marbillo
Irish Empire
Total War: Utopia (TWU)
Calone

Tau Empire information.[en.wikipedia.org]