When attacking a planet I got this message:

    Political Advisor
"We are at war with these scoundrels. Our forces will fight with greater strength!"

Ground against air 
2 laser turrets got annihilated from above by the attacking forces
Airfight 

There are no defending fighters on the planet
The final phase, groundfight
1 soldier died of stress in the attack
Attacker win!

We caught the empire on surprise, they probably did not expect an attack from an empire by a size so close to theirs. This did not give them any chance to destroy the buildings.
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