If an account is scheduled for deletion and you attack when it is being deleted your fleet will vanish off the screen as it has lost its target. The fleet will return to the planet you sent it from, taking the same amount of time that it has been underway for to return.
It is also possible for 2 or more alliance members to join forces to defend a planet or attack a target. This is called 동맹 전투 시스템 and it is currently only possible in Universes 5,18,19,20,21 and 22.
When you win an attack, you plunder the minerals on the planet.