Star Trek: 12 Biggest Losses Starfleet Ever Faced
9. Breen Attack On Earth
The Breen proved themselves to be both powerful and fearful in a very short time in Star Trek. The assault on Starfleet Headquarters, while not as devastating as the loss of Betazed or the later loss of the Chin'toka system, was a strike against the morale of the Federation alliance - one that is keenly felt throughout the quadrant.
General Martok sums up the situation perfectly: not even the Klingon Empire had ever dared to attack Earth. On that day, they learned something about the Breen.
Sisko comments on the loss of friends who had worked in the buildings that were now leveled. The need for revenge boils across the fleets - coming to a head in the second battle of Chin'toka.