By the year 19 BBY, Thrawn, a Chiss officer within the Chiss Expansionary Defense Force, launched a pre-emptive strike against a ground-based facility in the Unknown Regions. This facility was producing weapons in preparation for an assault on the Chiss Ascendancy. This pre-emptive attack was against Ascendancy law, and his actions were viewed as a breach of the Chiss doctrine of only retaliating when directly attacked. What was a secret to most was that Thrawn had orchestrated the entire situation, with the goal of being exiled and subsequently joining the Galactic Empire without any ties to the Expansionary Defense Force.
As a result, the Ascendancy banished the he officer to a deserted planet. There, Imperial Captain Voss Parck apprehended Thrawn and presented him to Emperor Palpatine, who then welcomed the Chiss into the ranks of the Imperial Fleet.
Peter M. Schweighofer initially alluded to this conflict in a biographical summary accompanying Timothy Zahn's short story, Mist Encounter, featured in Star Wars Adventure Journal 7. The revelation of its contrived nature, designed to engineer Thrawn's downfall, was detailed in Vision of the Future, the concluding part of Zahn's Star Wars: The Hand of Thrawn Duology.