Acts of nature I don't count as tragedies. They just happen and you deal with it. Certainly there was death, destruction, and pain on a massive scale but I wouldn't call it tragic. Its just how the natural world works. If you live in an area that is below sea level, expect to get wet.
Tragedies are man made. Tragedies are the result of human hubris. Tragedies happen when a person who should know he is doing something wrong and hurtful but does it anyway. Tragedies are the result of the failure of being human.
Also I did not say the re-election of George W. was the worse tragedy, but one of. Slavery in the US is in of itself one of the great tragedies that the US has done towards our fellow humans, but there are many tragedies in the history of the U.S.