John Rentoul is the biographer of a Labour leader. So am I. But I struggle to think what else we have in common.

Oh, wait! We both like to argue about Iraq, despite others having "moved on". Rentoul, like the subject of his biography, Tony Blair, was an ardent supporter of the invasion of Iraq; I, like the subject of the biography that I co-authored, Ed Miliband, happen to be an opponent of the war.

Yesterday, Rentoul and I found ourselves locked in a Twitter spat over whether or not the invasion exacerbated the threat from al-Qaeda-inspired terrorism -- not just to Iraqis but to the residents of the west and, specifically, the United Kingdom. At one stage, Rentoul remarked:

You won't get this bit, b/c you disagree with it so strongly: Iraq was right *even if* there were a risk of "radicalisation"

In fact, I was making the reverse argument: whether or not one supported the war, and there were good and bad reasons for doing so, it is undeniable that Iraq increased the terror threat and acted as a recruiting sergeant for al-Qaeda and its allies across the world. Like his guru Blair, however, Rentoul disputes this point and tried to cite the former MI6 chief Richard Dearlove in his defence. (Yes, the same Richard Dearlove who oversaw the production of dodgy dossiers on Iraqi weapons of mass destruction during his tenure as Britain's top spy chief and who, in July 2002, blithely told Labour ministers that in the US "intelligence and facts were being fixed around the policy".)

Rentoul also bizarrely claimed, in a blog post last night, that I had "lost" the argument with him, because I "used a Banned List phrase first: 'Blood and treasure'."

Ah, yes, the "Banned List". I must have been out of the country the day we all decided to put the chief political commentator of a Sunday newspaper in charge of the English language.

Putting his linguistic fascism to one side, I thought I'd actually address the substance of Rentoul's claims -- that Iraq did not radicalise young Muslims at home or abroad, that the evidence for such radicalisation is "thin" and that the threat from terrorism wasn't increased by the our invasion -- in greater depth, and with more detailed evidence, than is practically possible in 140 characters on Twitter.

So, let's see what the experts have to say about the link between Iraq and terrorism.

Here's the verdict of the UK's Joint Intelligence Committee, in a memo sent to Tony Blair in February 2003, a month before the invasion:

The JIC assessed that al-Qaeda and associated groups continued to represent by far the greatest terrorist threat to western interests, and that threat would be heightened by military action against Iraq.

Here's the verdict of a postwar study conducted by the Defence Academy for the Ministry of Defence:

The war in Iraq . . . has acted as a recruiting sergeant for extremists across the Muslim world . . . Iraq has served to radicalise an already disillusioned youth and al-Qaeda has given them the will, intent, purpose and ideology to act.

Here's the verdict of ex-MI5 boss, Eliza Manningham-Buller:

[W]hatever the merits of putting an end to Saddam Hussein, the war was also a distraction from the pursuit of al-Qaeda. It increased the terrorist threat by convincing more people that Osama Bin Laden's claim that Islam was under attack was correct. It provided an arena for the jihad for which he had called, so that many of his supporters, including British citizens, travelled to Iraq to attack western forces . . . And our involvement in Iraq spurred some young British Muslims to turn to terror.

Here's the verdict of another former MI5 chief, Stella Rimmington:

Look at what those people who've been arrested or have left suicide videos say about their motivation. And most of them, as far as I'm aware, say that the war in Iraq played a significant part in persuading them that this is the right course of action to take. So I think you can't write the war in Iraq out of history. If what we're looking at is groups of disaffected young men born in this country who turn to terrorism, then I think to ignore the effect of the war in Iraq is misleading.

Here is the verdict of the 2006 National Intelligence Estimate, ie the consensus view of 16 US intelligence agencies:

We assess that the Iraq jihad is shaping a new generation of terrorist leaders and operatives . . . The Iraq conflict has become the "cause celebre" for jihadists, breeding a deep resentment of US involvement in the Muslim world and cultivating supporters for the global jihadist movement.

Here's the verdict of the then director of the CIA, Porter Goss:

The Iraq conflict, while not a cause of extremism, has become a cause for extremists. Those jihadists who survive will leave Iraq experienced in and focused on acts of urban terrorism. They represent a potential pool of contacts to build transnational terrorist cells, groups and networks in Saudi Arabia, Jordan and other countries.

Here's the verdict of National Intelligence Council, the CIA director's in-house think tank:

Iraq provides terrorists with "a training ground, a recruitment ground, the opportunity for enhancing technical skills," said David B Low, the national intelligence officer for transnational threats. "There is even, under the best scenario, over time, the likelihood that some of the jihadists who are not killed there will, in a sense, go home, wherever home is, and will therefore disperse to various other countries." "At the moment," NIC Chairman Robert L Hutchings said, Iraq "is a magnet for international terrorist activity." According to the NIC report, Iraq has joined the list of conflicts . . . that have deepened solidarity among Muslims and helped spread radical Islamic ideology.

Here's the verdict of the former chief of the CIA's Bin Laden unit, Michael Scheuer, in an interview with me for our 9/11 special issue of the New Statesman earlier this month:

Iraq moved Osama and al-Qaeda from man and group to philosophy and movement. I don't think we've begun to see the disaster Iraq is going to cause in the years to come.

Here is the verdict of a report by the pro-war US think tank, the Brookings Institution:

The invasion of Iraq breathed new life into [al-Qaeda]. On an operational level, the United States diverted troops to Iraq rather than consolidate its victory in Afghanistan and increase its chances of hunting down Bin Laden. Today, al-Qaeda is reconstituting itself in the tribal areas of Pakistan. Politically, Iraq vindicated Bin Laden's argument that the primary enemy of the Muslim world was not the local Muslim autocrats, but the "faraway enemy," the United States.

Here is the verdict of Australia's Office of National Assessments:

A key judgement is that Iraq has been clearly used as a recruiting tool for terrorist groups around the globe with the number of jihadis steadily increasing.

Here is the verdict of the hawkish London think tank, the International Institute for Strategic Studies (IISS):

Invading Iraq damaged the war on terror, there is no doubt about that. It has strengthened rather than weakened al-Qaeda.

Here is the verdict of another respected London foreign-affairs think tank, Chatham House, reporting just 11 days after the 7/7 attacks:

There is no doubt that the situation over Iraq has imposed particular difficulties for the UK, and for the wider coalition against terrorism. It gave a boost to the al-Qaeda network's propaganda, recruitment and fundraising, caused a major split in the coalition, provided an ideal targeting and training area for al-Qaeda-linked terrorists and deflected resources and assistance that could have been deployed . . . to bring Bin Laden to justice.

Here's the verdict of the terrorism and al-Qaeda expert Rohan Gunaratna:

After al-Qaeda lost its Afghanistan base, it desperately needed another land of jihad in which to train and fight. Iraq has provided such a place . . . [T]he US invasion of Iraq increased the worldwide threat of terrorism many times over. Even moderate Muslims are angry about the invasion and post-invasion developments. This animosity toward the United States makes it easier for terrorist and extremist groups to continue to generate recruits and support from the suffering and grieving Muslims of Iraq.

Here's the verdict of a study conducted by al-Qaeda expert Peter Bergen, one of the few "terrorologists" to have met with Osama Bin Laden, and his colleague Paul Cruickshank:

Our study shows that the Iraq war has generated a stunning sevenfold increase in the yearly rate of fatal jihadist attacks, amounting to literally hundreds of additional terrorist attacks and thousands of civilian lives lost; even when terrorism in Iraq and Afghanistan is excluded, fatal attacks in the rest of the world have increased by more than one third. We are not making the argument that without the Iraq war, jihadist terrorism would not exist, but our study shows that the Iraq conflict has greatly increased the spread of the al-Qaeda ideological virus, as shown by a rising number of terrorist attacks in the past three years from London to Kabul, and from Madrid to the Red Sea.

Here is the verdict of Professor Robert Pape, the Chicago University political scientist who has studied every known case of suicide terrorism since 1980:

[I]n a broader sense, America has become perilously unsafe. Each month, there are more suicide terrorists trying to kill Americans and their allies in Afghanistan, Iraq, and other Muslim countries than in all the years before 2001 combined. From 1980 to 2003, there were 343 suicide attacks around the world, and at most 10 per cent were anti-American inspired. Since 2004, there have been more than 2,000, over 91 per cent against U.S. and allied forces in Afghanistan, Iraq, and other countries.

Here is the verdict of Marc Sageman, an expert on al-Qaeda and former CIA case officer, who has analysed the biographies of more than 500 terrorists:

This is especially true since the 2003 invasion of Iraq, which has inspired local young Muslims to strike out against the west. It seems clear that this invasion has created more terrorists in the west, refuting the thesis that "we are fighting them there, so we don't have to fight them here". The fact that these plots peaked in 2004, one year after the invasion of Iraq, provides empirical support linking the two events.

I mentioned Sageman and Pape to Rentoul on Twitter last night. As is so often the case with the hawks, when damning quotes, facts and empirical evidence are put to them, they dodge, evade and/or ignore. His response to me was:

I'm arguing with you, not Pape or Sageman.

Yet, only a few hours earlier, he had asked me:

What about engaging with Dearlove's argument?

So he is allowed to quote a discredited ex-MI6 chief, who is now employed by a firm with links to the Gaddafi family, and expect me to "engage", but I'm not supposed to quote Eliza Manningham-Buller, Stella Rimmington, the JIC, the NIC, the CIA, the IISS, Chatham House, Robert Pape, Marc Sageman or a study by Peter Bergen showing a "seven-fold increase" in al-Qaeda-inspired terrorist attacks in the wake of the Iraq war and then expect a response. Hmm.

To conclude, a reminder of the original question: did Iraq heighten the threat from terrorism and bolster al-Qaeda? I'm sorry to have to inform John Rentoul that this isn't one of those questions to which the answer is no.