At 8:46 EST on 11 September 2001 hijacked American Airlines Flight 11 was flown into the North Tower of the World Trade Centre in New York City. A second plane, United Airline Flight 175, hit the South Tower less than twenty minutes later. By 9:40 a third plane had crashed into the Pentagon in Arlington, Virginia, home of the US Department of Defence and less than four kilometres from the White House. A fourth plane came down in Shanksville, Pennsylvania, after passengers overcame the hijackers. These events marked the worst ever terrorist attack on American soil: 2,753 people died in New York, 184 in Arlington and forty in Pennsylvania. The events changed the international environment overnight; security became America's utmost priority and it lost all tolerance for countries posing any form of national security threat. Despite occurring across the Atlantic, the attacks were also felt in Britain. There were sixty-seven Britons killed, more than the Lockerbie bombing or the 7/7 attacks that would take place in 2005.

The hijackers were nineteen members of al-Qaeda and the US’ initial response to the attacks focused on Afghanistan, where the leader of the terrorist group – and mastermind of the hijackings, Osama Bin Laden – was believed to be hiding. On 7 October 2001, the war against Afghanistan began with UK and US aerial bombings. However, President George W. Bush soon turned his attention to Iraq. Iraq had long been a thorn in the side of the West, ever since President George H. Bush had called a ceasefire on the Gulf War and refused to depose the regime of Saddam Hussein. The terms of the ceasefire included an Iraqi obligation to destroy, remove or render harmless all of its WMD under the supervision of a UN weapons inspection team (the United Nations Special Commission – UNSCOM), but Saddam remained uncooperative and obstructive throughout the 1990s.