So yesterday's what victory cost too much question got me thinking. What defeat cost too much? I'll nominate

In addition to the death of Ace there was the death of Whitebeard. The Navy lost Garp and Aokiji due to this fight. The price for victory would have been massive, but the cost in defeat cuts even deeper.

My second nominee isn't from a show. It would have to be from the book the Fall of Reach. At least I think that is the correct novel. The one where most of the MkII Spartans lost their lives. It has been a few years since I read the books so you'll have to forgive me if I don't have the details just right.My third nominee is almost cliche and it wasn't even a fight. The Death Star blowing up Alderaan.