Operation Guardian, also known as Project Guardian, was a Starfleet military engagement that was orchestrated in the alternate War of the Prophets timeline which was tasked with combating the Bajoran Ascendancy and their plans of a merger between the blue wormhole and red wormhole Jalbador which would have destroyed the entire universe.
The operation was under the command of Admiral Kathryn Janeway who led joint Federation and Borg forces that clashed with the Grigari around the planet where the Guardian of Forever was located. The purpose was to secure the ancient device and use its time traveling capabilities to alter the timeline. The Operation saw fifty to sixty starships, which included several Borg cubes to regain the Guardian of Forever. The battle lasted for several days with the allied forces managing to land ground troops on the planet. However, just as they were within sight of the Guardian, the Grigari activated a Singularity bomb that destroyed the allied forces, the Guardian and even the Grigari themselves which ended another one of Starfleet's desperate plans to survive. (DS9 - Millennium novels: The War of the Prophets, Inferno)
- It is entirely possible that these were not a reformed peaceful Borg Collective but rather a splinter group of Rogue Borg such as the Independent Nation of Borg. Though it is possible that the Borg engaged in an alliance with Starfleet in order to prevent the universes destruction.