To clarify, if you have +9 defense and someone sends 10 ships, the map will play, but upon completetion of map (since can't do it at begining or it will be abused with !exit), 9 starships would be destroyed and you'd only get the bonus from 1 starship that survived.
Another example:
Player sends 14 starships
Planet has +9 defense.
Map plays
Once game is over
if player who sent starships wins, the planet destroys 9 starships, leaving player with 5 starships left over for bonus influence.
Mayhaps make it so that if the player loses he only loses 9 starships as well.
This change is just to make it so planets are being exchanged within just a single battle. Since you can destroy structures, you can also weaken the defenses, so even if you win, if you destroy planet defense structures, you will need less starships next game cause defense will go down to +6 defense.