The spectre of the 2003 Iraq War hung over the House of Commons as David Cameron sought to draw a dramatic distinction between the UK's imminent military action against Islamic State in Iraq, and "past mistakes".
With his hands noticeably shaking as he stood in the dispatch box to address the recalled House, the Prime Minister said there was "no more serious an issue" than deciding whether to commit British forces to the international effort to tackle IS and acknowledged the military effort would last "not just months, but years".
The PM said the shadow of the 2003 decision to join the US-led invasion of Iraq "hangs heavy" over today's vote but told MPs: "We must not use past mistakes as an excuse for indifference or inaction. We will play our part in destroying these evil extremists, we will support our Muslim friends around the world as the reclaim their religion and, once again, our inspirational armed forces will put themselves in harm's way to keep our people and our country safe."
MPs will vote on whether to back the Royal Air Force joining the US-led bombing campaign, but British air strikes will be limited to Iraq rather than IS strongholds in Syria.
Cameron insisted that the action he was proposing was different to the invasion of Iraq approved by Parliament in 2003. And he added: "This is about psychopathic terrorists that are trying to kill us and we do have to realise that, whether we like it or not, they have already declared war on us. There isn't a 'walk on by' option. There isn't an option of just hoping this will go away."
Attorney General Jeremy Wright had provided advice that there is a "clear legal basis for UK military action to help Iraq defend itself from Isil," Cameron said. "The Iraqi government has requested our help and given its clear consent for UK military action, so there can be no question about this."
Labour leader Ed Miliband acknowledged that there was "unease" about intervention following the 2003 Iraq War but said "we cannot simply stand by" against the threat of IS.
"As we debate this issue today, I understand the qualms and, for some, deep unease that there will be about this undertaking both in this House and in the country. Those who advocate military action today have to persuade members of this House not just that Isil is an evil organisation but that it is we, Britain, who should take military action in Iraq."
He said action against IS met his six criteria for intervention: there was a just cause, it was a last resort, there was a clear legal base, military action had a reasonable prospect of success, it had regional support, and was proportionate.
The Labour leader said the legacy of the Iraq War meant there was a "heightened responsibility" on the UK to support the country's government.
Quoting Robin Cook's resignation speech on the eve of the Iraq War, Miliband said the former foreign secretary and leader of the Commons had told MPs "our interests are best protected not by unilateral action but by multilateral agreement and a world order governed by rules".
A YouGov survey for The Sun shows the most staunch support yet for RAF airstrikes in Iraq, with 57% of the British public now in favour and just 24% against. And voters also support bombing IS targets in Syria, with 51% in favour and 26% against, though Cameron has ruled out such raids.
The PM said he personally approved of UK strikes against targets in Syria, but strongly hinted that Labour or the Lib Dems had suggested they would not back intervention there. "I do believe there is a strong case for us to do more in Syria but I did not want to bring a motion to the House today which there wasn't consensus for," he said "It's better if our country can proceed on the basis of consensus."
He added that he did not believe there was a "legal barrier" to action in Syria but he acknowledged the situation there was "more complicated" than in Iraq because of the civil war and the position of President Assad.
The Labour leader said he had reservations about any future move to target IS in Syria. "We will consider any further proposition, if the Prime Minister so chooses to come back with it," he said. But there was a question of legitimacy, he said, and although there was a "strong argument" that action in Syria may be legal, "it would be better to seek a UN Security Council resolution".
Cameron said the UK had a "duty" to take part in the fight against IS and could not "subcontract" protecting British streets from terrorism to others. "Britain has unique assets that no other coalition ally can contribute," he said. "The Brimstone precision missile which minimises the risk of civilian casualties and which even the United States doesn't have. We have our unique surveillance and intelligence capabilities.
"We have highly professional forces which are well used to working with their US counterparts. These are some of the reasons why President Obama made clear to me that America wants Britain to join the air action in Iraq."
"This is not a threat on the far side of the world," Cameron went on. "Left unchecked, we will face a terrorist caliphate on the shores of the Mediterranean, bordering a Nato member, with a declared and proven determination to attack our country and our people. This is not the stuff of fantasy - it is happening in front of us and we need to face up to it."
IS has "already murdered one British hostage and is threatening the lives of two more", plotted attacks which would have affected British tourists and were "a terrorist organisation unlike those we have dealt with before".
Cameron said there was "a strong case for us to do more in Syria" but recognised that it would require more work to secure Parliamentary approval of extending British involvement to that country.
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"Of course, some will say that any action you take will further radicalise young people," Cameron went on. "I have to say this is a counsel of despair. The threat of radicalisation is already here. Young people have left our country to go and fight with these extremists. We must take action at home, but we must also have a comprehensive strategy to defeat these extremists abroad."
America, Britain and others "are not contemplating putting combat troops on the ground," Cameron said, a move which the motion being debated expressly excludes. "There will be troops on the ground but they will be Iraqi troops, they will be Kurdish troops, and we should be supporting them in all the ways that I will describe."
Cameron said it was made clear to the Iraqi government that outside help was "conditional on you defending and protecting all of your people, and that must include the Sunnis in Iraq as well". He said he remained hopeful of support from "particular countries that may be able to encourage the Sunni tribes" to be involved in the fight and said Iran had a "role to play" in addressing the crises in both Syria and Iraq. The jury is still out on whether they will play that role but we should certainly be encouraging them to do that," he said.
Though the PM said that he was committed to bringing any case for further intervention before parliament, Cameron said he would certainly take action without approval if it meant preventing a massacre or protecting a key British interest. "I think it's important to reserve the right that if there was a critical British national interest at stake or if there was the need to act to prevent humanitarian catastrophe then you could act immediately and explain to the House of Commons afterwards. I'm being very frank about this because I don't want to mislead anybody."
Cameron admitted, after an intervention by Tory MP Zac Goldsmith, that more had to be done to address concerns about key Western allies such as Saudi Arabia, Kuwait and Qatar, "to persuade those that in the past might have backed organisations like Isil because they were seen as a Sunni Arab organisation that they made a terrible mistake and they should not do it again."
The debate comes as the Danish government announced it was joining the coalition, sending seven F-16 fighter jets to take part in airstrikes against the group in Iraq. Belgium are also debating their involvement in the coalition, while the Netherlands has already announced it will take part. No European countries plan to deploy in Syria.
Danish Prime Minister Helle Thorning-Schmidt said her government would send four operational planes and three reserve jets along with 250 pilots and support staff. The deployment will last for 12 months. She urged other countries to participate, too. "No one should be ducking in this case. Everyone should contribute," she said.