What happens if the imperial player retreats ships and ground troops to a system that has the rebel base still hidden with rebel units in it?

Its no different than if you moved there normally.

The rebel base is revealed and combat ensues.

14 hours ago, Jobu said:

Its no different than if you moved there normally.

The rebel base is revealed and combat ensues.

After discussing it with my game group, we believe that technically there are enemy units in that system, so it should not be a valid retreat system. Allowing this creates a situation where the rebel players take significant risks if they operate a fleet one system away from their hidden base, as retreat can be used as a "overrun" effect to check two systems in the same turn by the Empire.

Could the rebel player decide to reveal their hidden base and units there to at least block retreat to that system?

10 hours ago, grisix said:

After discussing it with my game group, we believe that technically there are enemy units in that system, so it should not be a valid retreat system. Allowing this creates a situation where the rebel players take significant risks if they operate a fleet one system away from their hidden base, as retreat can be used as a "overrun" effect to check two systems in the same turn by the Empire.

Could the rebel player decide to reveal their hidden base and units there to at least block retreat to that system?

There aren't units in the system until the ground troops land, so there's nothing restricting the move.

Instead of seeing it as a weakness, see it as a trap. Or leave a single unit in the actual system to prevent this. Or don't engage next to your home base...

As for revealing the base... the timing is tricky and unsupported by rules in the book as it stands. Probably reasonable to submit as a FAQ, but I'd say, for now, just talk it through with whomever you playing -- its an edge case, but makes no sense debating it when it comes up, better to ask before hand.

I know I'd play as in the Rebel player cannot declare their base, and that combat would ensue... At least until it is proven to be an issue.

The timings for revealing the Base has been errataed:

"If the Rebel player wishes to optionally reveal his base, he can only do so at the start of one of his turns of the Command Phase, before using one of his leaders."

The timing would be weird but I'd say that you resolve the current combat before going on to the new combat. That makes the most sense and avoids any problems.