A little outside help I think is an understatement. In retaking DS9, Dukat had Sisko right where he wanted him. If it weren't for the unexpected arrival of the Klingons, the Starfleet armada would've been obliterated.
Sure, in both instances he had a little outside help, but who says winning wars must be accomplished without luck?
...That and he was given charge of the Defiant, arguably the most powerful ship that Starfleet had at its disposal, and used her well.
And to stop the Dominion fleet from coming through the wormhole, he begged. Ugh.
Being given the Defiant is not much of an accomplishment. Of course Starfleet gave the most powerful ship to the position that needed it the most--DS9--and not the captain that was most capable.
If there's anything Sisko is responsible for, it's getting the first Defiant destroyed and Jadzia killed.