[@TheSovereignGrave]

Alright, how bout something along the lines of this. 5 years ago, the Commonwealth sent a fleet task group to one of the Rimworlds with the intent of convincing or coercing it to join the Commonwealth. However, they severely misunderstood The Rim as a whole, and when a sizeable Rim fleet showed up a few days later, the Commonwealth forces backed down.

Something like that. Timeline is also flexible.