Star Trek: 12 Biggest Losses Starfleet Ever Faced
4. Battle Of Cardassia
The final push of the Dominion War sees the Federation alliance finally invade Cardassia, pushing for the the central core of the Cardassian Union. However, despite the huge fleet of Starfleet, Klingon, Romulan and, later, Cardassian vessels that take on the Dominion in orbit of the planet, there has already been a tremendous cost.
The alliance fleet loses a third of the forces they brought with them to attack the Dominion, which is a potentially fatal blow. It is in fact only because of the Cardassians switching sides at the eleventh hour that they are not completely overwhelmed, though the Cardassians themselves then face a horrendous loss.
The Dominion, as punishment for the Cardassians' betrayal, begin leveling cities. Before the sun sets on the last day of the war - one third of the Federation forces have been lost, along with 800 million Cardassian citizens - and all because Gul Dukat was tired piloting a Bird of Prey.