Tanis Battles intensify

On Tanis the Tau and Imperium continued to scout out the planet with minimal resources in mid to late 03.022M42. The Imperium had established a small base on one continent and would regularly send out patrols to try to work out whether the Tau were serious about the planet or system, or whether their raids were merely disruption attempts. They were bolstered by a small force of Grey Knights, who were investigating the level of psychic disturbance in the native nomadic human population. Unaware they were being tracked by the Tau and sensing something mysterious of a psychic nature, the Grey Knights got embroiled in an inconclusive firefight, forcing both sides to withdraw.

Later, the Deathwatch chapter of space marines followed this up. Unknown to the Imperium the Tau had established a number of bases dotted around the world, hidden from view and shielded by stealth technology from human sensor equipment. The Deathwatch then found evidence from the local tribesmen that the tau intended to attack the prometheum refinery, recently constructed by the Adeptus Mechanicus to ensure self sufficiency for the expedition to Tanis Prime.

This was a deliberate ploy by the Tau, and as the Deathwatch moved to defend the refinery, a pathfinder team attacked the small band of astartes. The Tau lured half the deathwatch away with a feint attack, then rapidly moved in from a different direction, forcing the deathwatch to take cover under a hail of pulse carbine fire. Having suppressed the astartes, the pathfinders then planted demolition charges on the main storage tanks, utterly destroying them. By the end of 03.022M42 the position of the imperial scouting forces on Tanis Prime was becoming precarious.