The biggest mistake of all is that the federation should have had half of the fleet standing by at the wormhole and just blasted everything coming thru. Or they should have mounted a bunch of Quantum torpedo "gattling guns" just sitting there waiting. They had plenty of warning. It's called a bottleneck. Just ask those 300 Spartans!
I think it's official that waiting too long is Federation's 3rd major FUBAR of the war. It almost seemed as if Starfleet was too scared to do anything!
3. The Federation and Starfleet failed to act in time.
During the phase of the cold war with the Dominion, starfleet did take appropriate action by bolstering up DS9 and turning it into something of an armed fortress.
The Defiant was also permanently stationed there for the sole purpose of protecting DS9 and Bajor. However, when Cardassian joined the Dominion, and the Dominion began sending fleet after fleet into the Alpha Quadant, Starfleet just sat and watched.
Even when reports of ships being attacked by Jem Hadar or going missing, Starfleet still did very little other than just watch.
The Jem Hadar are coming through your side of the wormhole and through YOUR territory, and yet the only response is just to note it.
Sure, it is said Starfleet never fires the first shot, and will not start a war. The arguement could be 'hey, our hands were tied politically and ethically until the Dominion fired the first shot'...
But it seems to fall apart just on the violation of territory thing, let alone the ships being attacked.
Starfleet waited and watched while the Dominion built up a huge fleet, mostly because they were able to set up shipyards by the time the war broke out- major mistake.
It would have been interesting to see that idea- ships positioned at the mouth of the wormhole firing at the jem hadar as soon as they appeared, but what are downsides to this idea?