Thanks! The nuclear bombings were obviously very tragic, though if we take the view that a progress-positive culture is the main criterion for tragicness, the development of the bomb may have been a pretty good period, since it led to nuclear energy and other innovations.
Assuming the Great Stagnation hypothesis is true, whatever happened in the 1970s to slow down science could be said to be our 3rd most tragic moment. But it looks like our civilization is self-aware enough to avoid a full return to stasis, so fortunately we’re not quite there yet.
Basically the exact same thing I said a few days ago! Possibly the thing to do would have been to convince the Athenians to listen to Alcibiades just before the battle of Aegospotami, when Athens lost most of its fleet.