Your troops return to the Acropolis without attacking a hostile City or Emporium in the following cases:
1. Your opponent has activated Full Protection over their City.
2. Your opponent has relocated their City to another place on the global map.
3. The targeted Emporium had depleted before your Units arrived to capture it.
4. Your opponent lost control over the Emporium.
Please note that it is impossible to attack an Emporium which isn't controlled by any player.
Many players use a special tactic to minimize their losses: when the player who keeps Emporium notices that it was attacked several times, they recall their reinforcements, leaving it unclaimed.