9/11 is a reference to the date September 11, 2001, and it is used to refer to the terrorist attack carried out on that date against the World Trade Center in New York City, the capital of the United States in Washington, DC, and the Pentagon. The World Trade Center, also known as the Twin Towers, were two of the five tallest buildings in the world at the time, located in the financial district of Manhattan. On the morning of 9/11, two planes were hijacked by terrorists later identified with the al-Qaeda terrorist group, with one plane being flown into each of the Twin Towers. A total of 2,996 people were killed in the attacks, including 19 members of al-Qaeda. In Twenty Years Later, Victoria Ford is assumed to be among the 2,996 casualties of the attack, and the identification of her remains and the phone messages placed to her sister, Emma, reflect the destruction and chaos of the attacks.

9/11 is cited as the initial cause for multiple wars between the United States and Iraq and Afghanistan, as well as multiple policy decisions on the part of the United States government to increase surveillance with the intent to predict future attacks.