New York Twin Towers 11th November, 2001: 2977 deaths.
Resulting War on Terror (2001 to 2121)
Afghanistan: 7 000 US servicemen
8 000 contractors
100 000 civilians.
Iraq: 151 000 "violent" deaths.
Annual road deaths in US: 38 000
London Blitz: 43 000 civilian deaths
Korean War: 54 000 US Military
227 000 Korean Military
Uncountable civilian deaths
Vietnam War: 282 000 US and allies military deaths
415 000 civilian deaths.
European colonisation of the area of the US;
(aka the Indigenous Holocaust, 1492 - 1992)
at least 100m deaths of Native Americans by military and "cowboy" action and disease.
The Harrying of the North: (Yorkshire and the neighbouring shires) ordered by William the Conqueror: 150 000 deaths (about three quarters of the then population).
Reports claim that the "the World changed" on September 11th 2001.
It seems to me it is carrying on much as usual.
Disclaimer: even in these information-rich days it is difficult to get systematic and comparable figures for these atrocities. All the above figures come from what appear to be reliable sources via Google. Some may be wildly out and I apologise for that, but the possibility does not, in my view, alter the conclusion. I shall, of course, be happy to receive corrections.