September 11, 2001 brought the costliest man-made tragedy in the history of civilization to the United States of America.
On that Tuesday morning, nineteen anti-American terrorists hijacked four commercial airliners - two of them United Airlines, and two of them American Airlines - with the intention of crashing them into prominent United States buildings. They were devastatingly successful.
Summary of tragedy:

George Weiss Jr.
March 3, 2004