If formal declarations of war are still as irrelevant in the 24th century as they were since WWII, then no declarations of war were ever exchanged between the Federation and the Dominion.
If no declaration was issued by the Federation council, then what about the Federation President's authorization for war, the Federation President is presumally the Commander In Chief of Star Fleet (I don't know) did the council give him, or even have to give him, anything for the fleet to engage in what came to be a major war?
On October 2002 the US Congress voted in place "Authorization for Use of Military Force Against Iraq Resolution of 2002" (Public Law No: 107-243) authorizing the commander in chief to take the country to war. Did the Federation President recieve a simular authorization?
Or (as I asked in my OP) can Star Fleet do this all by it's self? Part of it's standing rules of engagement, the fleet defending the homeland, military treaties with foreign powers being created, order member worlds to increase their defenses, ships transferred to the main fleet, etc.