Capturing powerful bases: first you must get rid of their defending
fleet. The trick: Bases can only engage in a limited number of combats
per turn. One between movement phases 50 and 199, then one last time
at the end of the turn. If you hit the base early with a scout say on
movement phase 51 it will engage the base. Now any further fights that
develop between movement phases 52 and 199 will NOT include the base.
So you move your fleet in, engage the defending ships, then move AWAY
from the base before the end of the turn. Thus you do not need to
engage the base's ion cannon, AA guns or Home Guard wings, and the
base is open to capture by Ground Assault next turn.
I read this from
http://www.furfur.demon.co.uk/geop/v4startup.htm
it could be out dated by from what I can tell this site is pretty
good.
If this is a bug a possible "fix" would be a join combat switch.
but I could also see this as a feature sorta.
attacking Fleet Commander "Now that we know where the base, lets
approach from the other side of the planet"
still seems like this would be the avoid ground targets or a new avoid
ground base switch however.