Twenty years ago, the US and the UK announced that Saddam Hussein, the president of Iraq, was stockpiling weapons of mass destruction and supporting "terrorist groups".
On March 19, 2003, a US-led coalition began bombing Iraq, and one day later a ground invasion began. In the coalition were US, UK, Australian, and Polish soldiers.
In 2011, the coalition forces withdrew from Iraq, declaring the war over and leaving behind at least 275,000 people dead and a country ravaged.
Al Jazeera breaks down some of the war's defining moments in the following timeline.
Building up to invasion
After the September 11, 2001, attacks, US President George W Bush began claiming that Iraq had WMDs - a claim later proved false - and that it supported al-Qaeda, making disarming it a new priority.
On September 24, UK Prime Minister Tony Blair told the House of Commons: "Of course there is no doubt that Iraq, the region and the whole world would be better off without Saddam."
Hearing the intent behind those statements, hundreds of thousands of people around the world poured onto the streets to protest against a war in Iraq.
In November 2002, UN Security Council Resolution 1441 afforded Iraq “a final opportunity to comply with its disarmament obligations”.
Four days later, the Iraqi parliament voted to reject the UN resolution.