The first memorials to the victims of the September 11 attacks in 2001 began to take shape online, as hundreds of webmasters posted their own thoughts, links to the Red Cross and other rescue agencies, photos, and eyewitness accounts. Numerous online September 11 memorials began appearing a few hours after the attacks, although many of these memorials were only temporary. Around the world, U.S. embassies and consulates became makeshift memorials as people came out to pay their respects.

The September 11 attacks, commonly known as 9/11, were four coordinated Islamist suicide terrorist attacks carried out by al-Qaeda against the United States on September 11, 2001. That morning, 19 terrorists hijacked four commercial airliners scheduled to travel from the East Coast to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City, two of the world's five tallest buildings at the time, and aimed the next two flights toward targets in or near Washington, D.C., in an attack on the nation's capital. The third team succeeded in striking the Pentagon, the headquarters of the U.S. Department of Defense in Arlington County, Virginia, while the fourth plane crashed in rural Pennsylvania during a passenger revolt. The September 11 attacks killed 2,977 people, making them the deadliest terrorist attack in history, and instigated the multi-decade global war on terror, fought in Afghanistan, Iraq, and elsewhere.