The project took over two years to complete and came after the Prime Directive ceased to be in operation by the Federation. It involved the combined replicator construction of thousands of transporter pads that were modified to allow entry into the Mirror Universe.
The first goal of the Project was to actively enter the Mirror Universe and journey to Bajor where the fleet would re-enter their own universe and thus be behind Bajoran Ascendancy defense lines. They would then commence with the destruction of Bajor and remove the threat of the Ascendancy. Klingon forces were tasked with this operation and were led by Chancellor Martok who created the plan.
The second goal was to find a safe refugee for both states should the Bajoran Ascendancy's plan of unifying the two wormholes be successful and the universe was destroyed. It was theorized that the Mirror Universe would have escaped the destruction and would have allowed millions of refugees to rebuild should the first plan be unsuccessful.
Ultimately, the plan failed when the Grigari became aware of it and deployed assault fleets to destroy all resistance. One assault was made against the Federation territories during trade negotiations while another was deployed against the Klingon forces. (DS9 - Millennium novel: The War of the Prophets)