So I was playing Gal3 today,  and I tried to invade a planet with my transport, it was too weak to take it own, because they had a fleet of defending ships, so when i merged the transport with 3 escort videos and tried to move it over the planet (yes I had plenty of moves left) it did not work, it wont let me get past the defending ships, essentially it means any planet with defending ships is impossible to invade until they move their defending ships out.

I even tried to send fighters on their own to destroy the invader, woudl not work.