In the Voyager episode Scorpion the crew encounters Species 8472. They learn that the Borg started a war with them, but now the Borg are losing. Seven explains why they are so keen to assimilate 8472:
SEVEN: Species 8472 was more resistant than we anticipated. Their technology is bio-genically engineered. It is superior to that of all species we have previously encountered.
TUVOK: Which is precisely what you wanted.
SEVEN: They are the apex of biological evolution. Their assimilation would have greatly added to our own perfection.
However, the crew is able to create a weapon that proves to be most effective against 8472. They test it successfully and then move on, without giving the Borg the recipe for the weapon, and without being bothered by the Borg:
Captain's Log, Stardate 51003.7. Three days, and no sign of Borg or bio-ships. We appear to be out of danger, but the entire crew is still on edge and so am I. Not even the calm of Master Da Vinci's workshop is enough to ease my mind.
Now the Borg are at war with 8472, they are losing. Even if 8472 is now under the impression that the Borg could fight them and they decide to return to their fluidic space, the Borg will still want to assimilate them and therefore they will need the weapon.
So how come the weapon isn't the highest priority of the Borg? Why not send all available ships after Voyager? Are the Borg... grateful? Do they decide to honor their agreement after all? Or (to me) the most fascinating alternative: do the the Borg decide to make peace with 8472?!