Borg–Species 8472 War

Seeking new territory and technology, the Borg opened a passage into fluidic space, an extradimensional realm filled with organic fluid. There they met a powerful species they designated 8472, whose bio-ships and genetically resistant bodies could not be assimilated. Species 8472 counterattacked in the Delta Quadrant and destroyed entire Borg fleets and worlds.

In 2373 the USS Voyager found itself trapped between the two sides on its route home. Captain Kathryn Janeway struck a risky bargain with the Collective: in exchange for safe passage, the Doctor would develop a weapon based on modified Borg nanoprobes. A drone named Seven of Nine was assigned to work with the crew, and the new torpedoes forced Species 8472 to retreat into fluidic space in 2374.

The alliance collapsed as soon as the threat faded, but Seven of Nine, cut off from the hive, stayed aboard Voyager. A lasting truce with Species 8472 was later negotiated by Janeway.