To win the war, the Federation had to engage in lots of unethical and illegal activity. Wars are not won by following the rules.
For example, if Section 31 had not infected the Founders with the "can't change shape" virus, then Odo would not have been cured, and used that cure to convince the "female" Founder that solids were trustworthy, which allowed her to order the Dominion forces to surrender, instead of fighting to the last man, and causing enormous losses for the Alliance "victors".
I think the argument on this thread about WHAT kind of illegal and unethical activities the Federation "could have" partaken in is very silly. For the purposes of the story, the war needed to be a few seasons long, so that we could see the effects of the war on the main characters. All any of the techonobabbley weapons and strategies offered by some of the posters would have led to a few episodes of massive spacebattles and destruction, a few more to deal with the aftermath, and then a few more seasons of dull "alien of the week" stories or Bajoran religious/political disharmony stories. Meh
The only exception to this would be the phase - cloak. First of all we do not know if Starfleet would ever be able to stabilize the darn thing in the first place. Second, even if they were able to manufacture, mass produce, and mass install the cloak on all Starfleet vessels, it would still lead to the Romulans entering the war - but as the Federation's enemy. No phase cloak would be able to overcome that strategic disadvantage.