Your troops return into Harbor without attacking or blockading of a hostile Island or Emporia in the following cases:
1. Your opponent has activated Total Protection over his Island.
2. Your opponent has relocated his Island to another place on the global map.
3. The targeted Treasure Island had depleted before your battle squad has arrived to capture it.
4. Your units will return to the Harbor without attacking a Treasure Island if your opponent loses his control over it. Please note that it is impossible to attack a Treasure Island which isn't controlled by any player.
Many players use such a tactic to minimize their losses: when the player who keeps some Treasure Island notices that this Treasure Island was attacked several times, he recalls his reinforcements remaining it unclaimed.