On 18 March 2003, following one of the most momentous debates in modern parliamentary history, the House of Commons voted to authorise military action against Iraq. The decision gave prime minister Tony Blair the final approval to commit British forces to the imminent US-led invasion.

The motion was tabled despite the failure of the United States and Britain to secure a second United Nations Security Council (UNSC) resolution explicitly endorsing military action. The UNSC unanimously passed resolution 1441 in November 2002 to give Iraq “a final opportunity to comply with its disarmament obligations”. However, from November 2002 to March 2003, the UN failed to find any weapons of mass destruction after 700 inspections.

The United States and Britain later abandoned attempts to secure a “second resolution” in the face of opposition from France, Germany and Russia. The confirmation that Britain would abandon the UN route came as late as 17 March. US president George Bush responded by giving Iraq leader Saddam Hussein and his sons 48 hours to leave Iraq or face war.

Hours after this decision was taken, Robin Cook resigned from the cabinet as leader of the House of Commons. Cook delivered a famous personal statement to the commons, urging MPs to thwart the “commitment of troops in a war that has neither international agreement nor domestic support.”

But Cook’s departure did not spark a mass exodus of frontbenchers, as had been predicted by some commentators. Notably, the international development secretary, Clare Short, chose not to resign despite her public misgivings.

Also on 17 March, Lord Goldsmith, the attorney general, provided his final piece of public advice to parliament in a written answer on 17 March. The advice maintained that the authority “to use force against Iraq exists from the combined effect of Resolutions 678, 687 and 1441.”

Goldsmith added: “All of these resolutions were adopted under Chapter VII of the UN Charter which allows the use of force for the express purpose of restoring international peace and security.”

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Blair addressed MPs on 18 March to make the case for war.

He began by stressing that it was right for parliament to debate and vote on the issue. The choice before the House, the prime minister said, was “stark”: to stand troops “down now and turn back, or to hold firm to the course that we have set.”

He argued that the decision would determine the fate of the Iraqi regime, the future of the Iraqi people and the “pattern of international politics for the next generation.”

The prime minister argued that Resolution 1441 was “very clear” in laying down a final opportunity for Saddam to disarm. “Iraq continues to deny that it has any weapons of mass destruction”, Blair stated, “although no serious intelligence service anywhere in the world believes it.”

He condemned France for failing to “accept any resolution with an ultimatum in it.”

Blair noted that from 1998 to 2002, “no UN inspector was allowed to inspect anything in Iraq.” He insisted that the only reason Saddam had begun to offer concessions was the “imminence of force.”

The prime minister added: “The only persuasive power to which he responds is 250,000 allied troops on his doorstep. However, when that fact is so obvious, we are told that any resolution that authorises force in the event of non-compliance will be vetoed – not just opposed, but vetoed and blocked.”

He said it was tragic that the world had to re-learn the lesson that “weakness in the face of a threat from a tyrant is the surest way not to peace, but – unfortunately – to conflict.”

He warned that if the House demanded British troops be pulled back, Saddam would feel “strengthened beyond measure”.

Blair added: “What will the other states that tyrannise their people, the terrorists who threaten our existence, take from that? They will take it that the will confronting them is decaying and feeble. Who will celebrate and who will weep if we take our troops back from the Gulf now?”

The prime minister concluded by urging MPs “to give a lead; to show that we will stand up for what we know to be right; to show that we will confront the tyrannies and dictatorships and terrorists who put our way of life at risk; to show, at the moment of decision, that we have the courage to do the right thing.”

After around 10 hours of debate, the government’s main motion, which authorised “all means necessary to ensure the disarmament of Iraq’s weapons of mass destruction”, was agreed.

It passed by 412 votes to 149 – a majority of 263.

Meanwhile, an amendment declaring that the case for war had not yet been established attracted the support of 217 MPs. This figure included 139 Labour backbenchers, the Liberal Democrats, led by Charles Kennedy, and 15 Conservatives, including former chancellor Kenneth Clarke.

The Conservative frontbench, led by Iain Duncan Smith, supported the government motion.

In his speech, Duncan Smith argued that “when the government do the right thing by the British people, they deserve the support of the House, and particularly of the main opposition.”

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The invasion of Iraq – Operation Iraqi Freedom – began on 20 March 2003.

Blair notified the nation in a live televised address.

The prime minister stated: “On Tuesday night I gave the order for British forces to take part in military action in Iraq. Tonight British servicemen and women are engaged from air, land and sea.

“Their mission: to remove Saddam Hussein from power and disarm Iraq of its weapons of mass destruction.”

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On 28 April 2005, the confidential 13-page legal advice provided by the attorney general, Lord Goldsmith, to the prime minister in 2003 regarding the legality of the Iraq war, was published in full. The disclosure, which came at the height of the 2005 general election campaign, revealed the attorney general had privately counselled that a court “might well conclude” military action in Iraq was illegal without a second UN resolution.

Goldsmith’s advice warned that the “language of resolution 1441… leaves the position unclear”. He advised that while he could make a “reasonable case” for military action, “the safest legal course would be to secure the adoption of a further resolution to authorise the use of force”.

This private advice was not shared with cabinet. At the time of its disclosure, it was noted that Goldsmith’s final piece of public advice – provided to parliament in a written answer on 17 March 2003 – was unequivocal.

Blair denied that the private advice amounted to a declaration that the Iraq War was illegal, and defended the decision not to circulate it to the cabinet.

In 2009, Blair’s successor as prime minister, Gordon Brown, established an inquiry to investigate the run-up to the war and its aftermath. In a statement to the commons, Brown explained that, with the final British combat troops about to return from Iraq, it was “the right time” to ensure a “proper process in place to enable us to learn the lessons of the complex and often controversial events of the last six years.”

The Iraq Inquiry was officially launched on 30 July 2009.

Blair was publicly questioned by the inquiry on 29 January 2010, and again on 21 January 2011.

On 6 July 2016, the findings of the Iraq Inquiry, chaired by Sir John Chilcot, were published after seven years of investigation. The 2.6 million-word report – longer than the King James Version of the Bible, the complete works of William Shakespeare, and Tolstoy’s War and Peace put together – provided a searing indictment of the intelligence failures, shaky legal rationale, miscalculations, inadequate military preparation and exaggerated public statements that led to the UK’s involvement in the 2003 invasion of Iraq.

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The 18 March 2003 vote marked the first time MPs had been given the opportunity to signify a collective position on, and issue an effective veto of, military action.

Constitutionally, the power to declare war is exercised by the monarch on the advice of the prime minister as a royal prerogative power. Before the vote authorising military action in Iraq, governments had assumed the right to exercise this power to deploy troops – including ahead of the Falklands War (1982), the First Gulf War (1991), and the Kosovo War (1999).

The Iraq War vote therefore signalled the emergence of a political convention, granting parliament a veto over the deployment of troops. This precedent was effectively affirmed by David Cameron as prime minister, who held votes on military action in Libya (2011), in Syria (2013), and in Syria again (2015).

Cameron’s government lost the 2013 vote, which was considered binding by the then government.

However, in April 2018, Theresa May as prime minister authorised military airstrikes against Syria without first seeking parliamentary consent. May’s actions ultimately cast some doubt on the shape of the convention that had developed since 2003.

But her decision to pursue military intervention without parliamentary approval appeared to rely on possible exceptions to the convention, allowing for the government to act at speed.

In a written statement issued in 2016, the then defence secretary, Michael Fallon, declared: “In observing the convention, we must ensure that the ability of our armed forces to act quickly and decisively, and to maintain the security of their operations, is not compromised.”

The May government later won a symbolic vote on whether the House had sufficiently debated the matter of Syria by 314 to 36 votes – a majority of 278.