They haven't entered any of my systems. Instead, they either entered or spawned in a neighboring AI-controlled system, and are currently pathing very spastically between the wormholes, like the starships that sometimes spawn on AI worlds except drunk.
Since you mention "achieving their objective" I suppose I should mention that they wiped out a force of resistance fighters that heroically charged in when they showed up, just before this autosave. If that constituted achieving an objective then it probably shouldn't have though.