Intimidation

The counter-invasion didn't stop there. The Chapters that had launched the invasion had been given orders to teach the tau a lesson, to leave a scar on their collective psyche so deep they would fear straying into humanity's realm for centuries to come. Making translation to the warp once more, the Space Marine fleet crossed the Damocles Gulf with relative ease and burst back into realspace in the heart of sovereign tau territory. This time the tau navy put up a fight, but against the capital ships of a crusade fleet, they were still found wanting.
…
When it came to giving the tau a much-needed revelation concerning the true size and power of the Imperium, 'Operation Pluto' was a resounding success. We were not able to finish the job, however. News of the Tyran bio-fleet heading towards Ultramar forced us to withdraw. We left before Dal'yth's military heads, Commanders Farsight and Shadowsun, could be slain.

Farsight: Crisis of Faith