Man-made disasters: Hiroshima/Nagasaki
Although the destruction of these cities did end the pacific war, I don't think the killing of tens of thousands of innocents civilians was the way to do it.
The overall destruction and aftermath should be a lesson to all world leaders about the horror of nuclear weapons.
9/11:The Twin Towers
One of those days that I won't forget. It showed that war doesn't need to be fought on a battlefield, nor does it have to follow the rules of engagement.
The destruction of the twin towers was more to do with showing the fallability and complacency of the USA, than the mere destruction of a building and the killing of people.
Asia: Tsunami
A natural disaster the like of which I have never seen before, or want to see again.
The real horror of this is that it can happen again... and there is little we can do about it