YES, I believe it would be immoral! If a "Hostile Country" occupied this country. I would imagine that we would be busy attacking them from within what was "our" country.
If while being "Occupied" we still had the ability with hidden air bases to hit "Their" country's military targets then that would not be immoral. Of course during times of war we do attack opposing towns and cities since it is war.
We did bomb German and Japanese civilians but that was during a war when they were trying to take over other countries. We were not trying to take over any countries when this attack against our civilians took place. For that matter we are still not trying to "Take Over" any countries, we are just seeking retribution for terrorist acts.