I would argue that the Feds declared war implicitly by mining the wormhole, which was in itself an act of war.
The Bajoran government and the Federation had every right to place mines in their terrority. If they had placed mines in Cardassian space, or in Dominion space in the Gamma Quadrant, that would have been an act of war.
This was simply an act of internal defense. All they were doing was telling the Dominion that they couldn't pass through Bajoran space and they set up a means to make that possible. In effect, they were fortifying their borders, which every state has the right to do.