That is true, but sometimes the whole thing seems unfair and uneven. If I'm not mistaken, Sisko went out of his way to contact Cardassia first, then reacted when Dukat asked for help.
What makes The Fed (or even US) decide to intervene militarily in some affairs, yet ignore or do little in others?
Politics. Pragmatic considerations. The Cardassians could make good allies, etc. As noted before, DS9 was a deconstruction of the Trek philosophies. The Federation was just being practical.