I find it really dark how there is no talk about doing the same to the Klingons and Romulans (and indeed, plenty of talk supporting further befriending of these powers) even though both Klingons and Romulans are in their own way as bad as the Cardassians - and just as opposed to the values of the Federation (to the point that part of the reason they are fighting each-other is to enable the winner to better resist the Federation). Why are the Cardassians somehow less deserving of peace, security and planets without glass surfaces than these powers who have actually fought to destroy the Federation and killed Federation citizens by the billions? The Cardassians (so far) have been far better neighbours.
I think this doesn't quite address the situation as the Federation sees it. Yes, there were horrible, horrible fights against the Klingons and Romulans. Yes, the Cardassians have not hurt us nearly as much. And yes, the Klingons and Romulans both have parts of their society that we really are opposed to.
But unlike the Cardassians, we have opened lines of friendship and communication with them. It is still very much a work in progress, but there is an exchange of ideas and understanding between us and the Klingons and the Romulans. Befriending these powers is, in fact, the Federation strategy. Why? Because it saves lives and, to take the cynical view, it applies social pressure. We're shoving data about our altruistic utopian paradise society with its (insert appropriate virtues here) ways into their hands and, well, that means they are having to come to terms with the fact that the Federation exists and works really quite well. And that does matter. I don't know if the Remans, for example, know how the Federation works yet, but I guarantee that they will, and that will probably someday turn into yet another crisis for Oneiros to shove on our plates, but it will be opportunity as well as danger.
Becoming friends with them is our super not-secret social manipulation strategy.
Why are we treating the Cardassians so differently then?
Because the Cardassians have caught on to our super not-secret social manipulation strategy. And it TERRIFIES them. They're worried about Starfleet, but the Obsidian Order has night terrors about black-ops FDS agents getting one of our diplomancer gods an unedited interview on the Cardassian Daily News and slipping it past the censors. Cardassian society is a house of cards, and while perhaps the center might hold (for a time), the outskirts are all too easily torn away. They demanded we stop talking to their minion species in the Treaty of Celos because that's all it takes for the Federation Memetic Hazard to wreak havoc.
It's not like the Klingons and Romulans, where we had to get them to acknowledge our strengths and virtues to pry open a dialogue. The Cardassians know those things, and that's why they are refusing to open a dialogue. All of this discussion is about how we can disrupt their belief in this strategy enough to initiate plan Friendship.