Star Trek: 12 Biggest Losses Starfleet Ever Faced
10. The Battle Of Betazed
The episode of Star Trek Deep Space Nine In The Pale Moonlight is often seen as a game changer, not just in this series but in the franchise as a whole. It is easily the darkest episode of franchise, at least until Star Trek: Discovery began airing.
Sisko embarks on a dangerous plan to bring the Romulans into the war, based on the continued losses that the Federation are facing against the Dominion. There is a moment of doubt, however, where he worries if the plan has enough merit.
The Dominion answer the question for him, invading Betazed - one of the core worlds in the Federation. They take the Tenth Fleet completely by surprise, managing to secure the planet within ten hours.
While the loss of the planet alone is enough to justify a place on this list, it is also the strategic power that it gave to the Dominion. They are then well-placed to attack Vulcan, Alpha Centauri, Andor and Tellar.