I don't think it was really a matter of not conquering anyone. They could have just destroyed the Cardassian military without out-right conquering Cardassia - and since I assume the Cardies attacked first destroying their military power so as to prevent further attacks would have been still a defensive act. It was more that the Federation decided that that result wouldn't have been worth the increased casualties and destruction caused by an all-out war. I have a feeling the Cardassian War was already a rather unpopular war in the Federation (something like Vietnam for Americans). And in the latter stages of the war the Feds had a lot more to worry about on their other borders, what with the Borg, the Romulans coming out of their isolation, the Klingons in turmoil, etc.