On September 11, 2001, about 3000 people, mostly Americans, died on US soil from hostile action. According to many, this was an incomparable catastrophe that "changed the world".
On September 17, 1862, over 3600 Americans died on US soil from hostile action at the Battle of Antietam. Another 3600 died later from wounds recieved that day. In the Civil War althogether about 620,000 Americans were slaughtered by hostile action on American soil.
In October of 1918 and the few months following, over half a million Americans, mostly civilians, died rather suddenly from the Spanish Flu, a gift of WWI.
On August 6, 1945, an American bomber dropped the most horrible weapon yet known on a schoolhouse in Hiroshima, Japan. American hostile action that one day murdered 100,000 noncombatant men, women and children. A few days later, we poisoned, burned, blasted and melted to death an equal number of perfectly innocent people at Nagasaki.
Changed the world? If only.