Oh, there's no doubting the intent of the Dominion when first contact occurred. But, I refer you to the events of DS9: "The Ship." The episode ends with the tragic realization that Sisko and Kilana were each sincere in their offers, but that the deaths of Sisko's people and Kilana's Jem'Hadar - not to mention the injured Changeling in the titular ship - were the results of Sisko and Kilana being unwilling to trust each other.
What if Sisko and Kilana had trusted each other? Could this not have been the beginning of a kernal of understanding between the Federation and the Dominion? The springboard for the Federation convincing the Dominion of its sincerity in peaceful exploration and perhaps bringing the Founders closer to the realization that the Female Changeling finally reaches at the end of the war; that not all solids are the enemy?
Yes, the Dominion struck first and destroyed the Odyssey. But, the Klingons also struck first and destroyed many starships, including the Grissom; yet, peace was possible here.