This action left the Federacy in particular vulnerable. Should Chaos or the Imperium gain control of the Foramen Majoris, or worse, have it destroyed, the Federacy would be on the front line against a myriad of foes, with less than enthusiastic support from the Tau Empire. The Aeldari, particularly the Rillietan, had seen the threat, and the possible outcomes of such Tau disinterest, and decided to act.
Rather than propose an alliance, which they knew would be difficult and uneasy at best following the events in the Perseus Deeps, the Harlequins decided on a more direct approach. To ensure the Tau were ready and their minds focussed, the Tau would need to be hit, and hit hard on their own territory. To make their point the Aeldari prepared a force and conducted a devastating raid on Protogonus on 0212.017M42.
The destruction wrought by the one time allies of the Tau appalled the alien race, who appealed to the Federacy for aid for their furthest flung colony. The Federacy, quite rightly, rejected this request, citing that too many had already been lost in the unsuccessful Destino campaign. Skyfall then intervened, sending his own forces to Protogonus and planning a significant fortification of the colony. Despite his forces initial inability to defeat the Eldar, the Harlequins then withdrew of their own accord, having achieved their objective.