I doubt that either side cared much about formal declarations of war here. To paraphrase the Borg: "Declarations of war are irrelevant. You will be assimilated."
And when your opponent doesn't care why should the Federation council?
I would also like to point out that formal states of war (and/or declarations of war) have been increasingly become irrelevant in the real world since World War II. For instance, the period between 1941 and 1945 was the last time the United States had been officially
at war. Korea, Vietnam, Iraq (both times), and Afghanistan were merely prolonged cases of "military action" without actually declaring war on anyone. On the other hand, North Korea and South Korea have been in a continuous state of war since 1950, with a mere cease-fire being in place for most of the time between 1953 and 2011.
The bottomline is, whenever the Federation encountered a Borg ship, they tried to open fire on it (and the other way around). At the very least, I'd call that a de facto
state of war. The only limiting factor is that the core territories of both opponents were separated by 50,000 light-years or so. As result, fights occured only sproadically with very long break between individual battles.