Orks defeated in Mabb Nebula

In mid 05.0143M42, the tau realised that the continued ork victories on Hylas, while weakening the imperium, also posed a threat to their own ambitions in the Zadoc subsector. Were the orks victorious on Hylas, they would likely turn on the tau holdings in the Mabb Nebula, which put the entire aleph sphere expansion under threat. Rather than directly help the imperium, whith whom the tau were at war with on Tyranteous. Instead the ethereals determined that Fort Aerin, a powerful and important ork occupied base near to Va Doran, should be rendered ineffective. This would deter the orks from attacking tau worlds while not helping the embattled general Van Dorn on Hylas with his greenskin enemy.

Despite the war on Tyranteous the tau force to carry out the attack on Fort Aerin still had imperial allies, in the shape of the Knights of Dorn. This astartes force had been working with the tau for some time and recognised the dangerous shared enemies of human and tau, and had long ago decided that this far on the eastern rim, co-operation with the tau was a neccessity if either race was to survive. Pointedly the chapter forces had stayed well away from Zadoc command since re-appearing in the Zadoc subsector.

The raid would have been an unremarkable event in sector history, but for the strange appearance of the eldar at fort Aerin - fighting on the side of the orks. Though this perplexed the tau especially, the eldar had forseen that were the tau successful on Aerin, the orks may be weakened sufficiently to abandon their dogged defence of fort Sparcos against the tyranids. This would - eventually - threaten the eldar exodite worlds once more. With a small intervention now, the whole crisis would be averted.

Unfortunately the eldar chose not to even attempt to explain this to their erstwhile tau allies, and were unaware of the astartes allies in the tau forces. This surprising lack of foresight was a tragedy for the eldar, who found their forces outgunned and outmanouevered. Despite the usual violence expected from greenskins, the orks were defeated and fort Aerin ransacked by the tau forces, securing the tau right flank in the Mabb nebula and souring tau-eldar relations in the subsector.