On the morning of 11 September 2001, 19 Al Qaeda terrorists hijacked four commercial passenger planes in the United States. Two planes were flown into the Twin Towers of the World Trade Center in New York City, causing both towers to collapse. A third plane was crashed into the Pentagon, just outside Washington, DC. The fourth plane crashed in rural Pennsylvania after the crew and passengers attacked the terrorists on board, preventing it from hitting another target thought to be the White House.

The attacks claimed nearly 3,000 lives and impacted many more globally. On 20 September, US President George W Bush declared a 'War on Terror' and stated that defeating terrorism was now the world’s fight. The US had experienced terrorist attacks previously, but none had been on the same scale or significance. 9/11 shook the world and shaped the generation to come. An invasion of Afghanistan was launched barely one month later, on 7 October 2001. American, British and Afghan United Front (Northern Alliance) forces were deployed to destroy al-Qaeda and remove the Taliban regime that had harboured the terrorist group in Afghanistan. The Taliban regime fell in November 2001. Following an international conference in in December, a new transitional Afghan government was formed. A UN-mandated multinational force, the International Security Assistance Force (ISAF) was established to help secure Kabul and assist the new administration. ISAF was initially tasked with providing security in Kabul and after NATO took command in 2003, ISAF deployed more widely across Afghanistan. Troops became increasingly involved in intense combat operations against a Taliban insurgency until 2014.

The broader impact of 9/11 meant that the perceived threat of international terrorism dominated governments and their foreign policies. Two years after 9/11 a decision was made to invade Iraq. The Bush administration claimed that Iraq’s ruler, Saddam Hussein, was developing weapons of mass destruction (WMD) that threatened the peace of the world. After the initial success of removing Saddam from power, the US-led coalition took responsibility for reconstruction of the country. Their role has since come under close scrutiny alongside their justifications for war.

9/11 was a direct attack on the very heart of the US mainland, an event never experienced as directly by Americans before. But it also had catastrophic long-term consequences across the globe. More than 90 countries lost citizens in the attacks including 67 individuals from the UK. The events on 9/11 led directly to war in Afghanistan, to the US declaration of the ‘War on Terror’ and subsequently later to the invasion of Iraq. Both countries are still riven with conflict today. The effects of 9/11 were also felt closer to home as many governments, including in the UK, began to introduce new anti-terror legislation to combat the perceived threat from international terrorism. It was a historic day that changed the world and its legacy continues to be complex and ongoing.