it doesn't take two to start a war. See WWII for example.
I don't see what the UFP is supposed to do here if not fight other than destroy the wormhole(which would be a disaster for Bajor) or just surrender.
The Dominion were supremacist conquerors. Once the wormhole was discovered, conflict with them was pretty close to inevitable.
Also, it wasn't Odo's "compassion" that ended the war, it was that he was a changeling and his offer to provide the cure.