The so-called Island Seizure Battle is more like a tournament.
The difference is that a typical tournament is one-on-one, but this one is thirty-on-thirty. In a typical tournament, the fight might stop at a certain point, but here, it's a fight to the death.
The thirty contested islands are like thirty arenas. Once one side wins, they cannot pursue the other.
After capturing an island, the opposing side cannot attack for five days. Conversely, if the defending side wins, the other side cannot attack for five days.