The war on terror, officially the Global War on Terrorism (GWOT), is a global military campaign initiated by the United States following the September 11 attacks and is the most recent global conflict spanning multiple wars. The main targets of the campaign are militant Islamist movements like Al-Qaeda, Taliban and their allies. Other major targets included the Ba'athist regime in Iraq, which was deposed in an invasion in 2003, and various militant factions that fought during the ensuing insurgency. After its territorial expansion in 2014, the Islamic State militia has also emerged as a key adversary of the United States.

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.