The Borg. We've never seen the Borg at war except with Species 8472. It's always been a piecemeal battle with in-universe forces demanding one or two cubes at a time for victory. War means an all out battle with the end being dominance of another system, or quadrant. If the Borg had chosen to commit forces with the end being the destruction of the AQ, or GQ, rather than assimilation and acquisition of it's humanoids and technology, it would have happened.
The writers couldn't give the Federation more of a fight than it could overcome, the show must go on. I can't realistically imagine 100 or more Borg Cubes and dozens of Spheres, which they could have easily mustered out of thousands upon thousands of ships, not overcoming any armada the Dominion could field.