I am playing a game the dreadlords have become active.

I am finally at a point to move against them but I cannot invade their planet. 

I assault with a combat fleet (several) to wipe out defenses.

When I attempt to invade with transports, I get "planet is defended. Destroy defenses with attack ship(s)."

I try attacking with attack fleets, nothing happens.

If I combine attack ships into the same fleet as the transports, nothing happens.  

Is there some game mechanic I am overlooking?