To be fair, Maxwell destroyed a single warship
Well, after it had received the Free Shot Card and done its worst against an unshielded Phoenix. And it wasn't as much a "fight" as it was a "weapons release": a single volley of torpedoes did in the warship.
But admittedly only after the alliance with the Dominion, possibly including very significant weapons upgrades. Although such potent fixed weaponry being standard for all powers would go a long way in explaining why interstellar wars aren't over in two hours.And later on, we see in DS9 that Cardassian Weapon Batteries (not Dominion ones) can shred top of the line Fed ships.
There was no mention of there having been a war ongoing with the Klingons in "Errand of Mercy" - not ever, and certainly not during the previous episodes of the show. That is the key difference wrt the Cardassian case, I guess.Anyways, it's not much of a problem to have the Cardies just appear like this. TOS did the same thing with the Klingons, there was no mention of them before "Errand of Mercy".
With Romulans, there was mention of a war but also explication that the war had been over for a full century, making it even more distinct from the Cardassian case.
Timo Saloniemi