Borg Civil War

The Borg Civil War was a conflict between the Borg Collective and Unimatrix Zero, a rebel faction led by individual Borg, backed by the USS Voyager.

Discovery of Unimatrix Zero
Unimatrix Zero was formally discovered by the Voyager crew while Seven of Nine was regenerating. She reported her discovery to Captain Kathryn Janeway who decided to answer their call for help.

The Borg's Crackdown
Unfortunately, the Borg Collective had also discovered Unimatrix Zero and the Borg Queen ordered a crackdown. Thanks to a Vulcan mind meld by Tuvok. Janeway and Seven saw the Borg's invasion of Unimatrix Zero. The Borg had reassimilated dozens of inhabitants in order to stamp out the resistance, but Janeway began to hatch her plan to help the inhabitants.

The Plan
Janeway, B'Elanna Torres, and Tuvok allowed themselves to be assimilated by the collective after receiving inoculations to help resist assimilation. Once aboard a borg vessel, they introduced a virus to its central plexus, where it infected the Collective, allowing members of Unimatrix Zero to retain their individuality outside of their regeneration cycle.

Destruction of Unimatrix Zero
The Borg Queen continued her efforts to cleanse the Collective of infected drones, threatening Janeway with the termination of millions if she did not convince them to surrender their self-control back to the Collective. She began engineering her own virus to wipe out all drones connected the Unimatrix. In reaction to this, the Voyager crew begins disrupting the Unimatrix interlink frequency, causing the permanent collapse of Unimatrix Zero, but leaving behind many drones now free of the control of the Collective.