Deep Space Nine may have been the Best Trek Series but it was the worse at stringing together it's expanded story.
No one thought out the Dominion War Story so it was rather thrown together and the pieces don't fit at all.
There have been numerous explanations to try an explain all these issues:
-How and when did the Federation over come the shield issues Odyssey had in the first encounter?
-How did the Federation win a war with no major Fleet Victories.
-Why did the enemy retreat from DS9 when they out numbered Star Fleet 2 to 1? An even fight still favors the Dominion in Fire Power.
-How did the Klingons hold off 30,000 enemy ships with only 1,500? (and squandering them)
-If the Klingons can deploy planetary defense grids around captured Cardassian worlds why doesn't Bajor have one from the Federation?
-If the Federation had a year to rearm DS9 to repel a 50 ship fleet then why didn't Earth and Betazed have the ability to repel their respective attacks.
-If Earth has a defense system as said in Paradise Lost (where was it in the attack)?
-Why don't we ever see a Federation ship aside from Defiant fire on a Dominion vessel in 3 years after Odyssey was attacked?
*and my personal favorites*
-Where the hell are the shields in all these battles!
-Where the hell is the Enterprise in these battles!
-Where are Star Fleets UBER ships (Sovereign and Prometheus)?
-If the Dominion Battleship was such a problem (for one the most powerful warships in the quadrant) why no other stories about defeating it?
My explanation is a mixture of classical Berman Blame and poor concept structure for the War Arc. The Xindi War seemed better assembled than this. Some in the Bridge Commander community say the Breen had cloaking devices (it is inferred in canon) and that's how they got through. Books have said there was an Dominion Operative on Earth that disabled the Planetary Defenses prior to the attack and that fighters were the cause of what little damage we saw. But don't look for any consistency from the narrative.
Much of the Dominion War Arc that we saw was more for Dramatic Effect than actual story telling. They needed the War to look dire so they had a fleet completely destroyed. Federation ships like Odyssey were never given a fair fight and engagement but they had numerous miraculous victories with not much in the way of tactics to support it. They stalled the War for a Year and once it began just shy of half of each of those season had nothing to do with the war at all...even to the point of sparse on dialogue. And to rap it up quickly instead of a decisive action by the allies the invented disease earlier was given a cure to offer to end the blood shed. As plot devices go these aren't bad at all but it's the total view that has a lot of missing pieces.
So if you're expecting canon to tell you something specific (especially in DS9) like whether a Starbase should have weapons, you might be disappointed. It depends on the plot.