The Borg's invasion of fluidic space began a war with Species 8472 that Voyager helped end by giving the Collective a nanoprobe weapon in exchange for safe passage.
The Borg forced an opening into fluidic space, where they met Species 8472, beings immune to assimilation who struck back with powerful bioships. In 2373 Voyager reached Borg space and found destroyed cubes. Kes sensed through telepathy that 8472 intended to wipe out all life in the galaxy.
Janeway made a deal with the Borg. The Doctor had found that modified Borg nanoprobes could destroy 8472 cells, and Janeway offered to develop weapons from them in exchange for safe passage through Borg space. Seven of Nine was assigned as the Borg liaison aboard Voyager.
The weapons drove Species 8472 back into fluidic space early in 2374. The Borg tried to break the deal, but Voyager cut Seven off from the Collective and escaped. In 2375 Voyager found 8472 agents training inside a replica of Starfleet Headquarters and negotiated a truce with them.