Rogue Changelings, evolved from the Dominion War’s Section 31 experiments, infiltrated Starfleet at high levels. Working with the Borg Queen, they used DNA-altering technology to infect young Starfleet officers with Borg nanovirus modifications.

In 2401, on Frontier Day n 2401, during what was meant to be a celebratory fleet demonstration for the United Federation of Planets, they took control of the new Fleet Formation system, which allowed starships to synchronize navigation and tactical functions. The Borg signal seized control of every assimilated officer under the age of 25. The entire armada, now controlled by the Borg, instantly turned its weapons toward Earth and any ships resisting assimilation.

With Starfleet compromised, only a handful of unconnected vessels, including the USS Titan-A crew and the restored USS Enterprise-D, were capable of resisting the takeover. They traced the Borg signal to an immense transwarp node built into Jupiter. The Enterprise-D destroyed the Borg transmitter, freeing the assimilated officers from control and breaking the armada’s coordinated assault. The remaining Changeling agents were soon exposed and neutralized. It resulted in the destruction of the Borg Collective and its queen

In the aftermath, the Frontier Day catastrophe forced Starfleet to reevaluate its technological dependence, security protocols, and lingering moral failures stemming from past covert programs. Though the event ended in victory, it marked one of the most devastating internal attacks in Federation history and reshaped Starfleet’s strategic philosophy for decades to come.