There is a moral question involved if the Federation was at war with Cardassia at the same time the occupation of Bajor was happening.
There's that old saying about those that stand by and do nothing while evil happens....
This would make what Keeve Falor said even more important;
You are innocent bystanders, and I cannot condone violence against those who are not our enemies.
PICARD: Then I don't understand why you are unwilling?
KEEVE: Because you are innocent bystanders. You were innocent bystanders for decades as the Cardassians took our homes, as they violated and tortured our people in the most hideous ways imaginable, as we were forced to flee.
If they were at war and they knew the Cardassians were brutalizing the Bajorans, and the Bajorans were screaming for help, then why couldn't they help liberate Bajor.
There has to be some
inter-quadrant humanoid rights laws that all planets agree to in the 24th century
Plus, the Cardassians were always plotting to 'return' if the Federation ever left, even though it was painfully clear the Bajorans did not want them around in any capacity whatsoever.
If they returned, how exactly were they going to annex Bajor again so it would look legal?