The first question to ask is whether bombing Syria would worsen the terror threat in the UK at all.  Some will doubtless argue that precision strikes, aimed at ISIS ringleaders, would actually lessen the likelihood.  Michael Fallon, the Defence Secretary, was strongly supportive of such action against them in Raqqa, back in September, on the ground that plots for atrocities on British soil are originating there.  But the history of our engagement with Islamist extremism since 9/11, and more particularly since the Iraq War, suggests that foreign policy is an element in motivating British Muslims to bomb their fellow citizens.  So the sober answer to the question that opens this article is that, yes, it is likely to increase the likelihood of attacks with mass casualties on our own soil.  And the answer to the question in the headline above it is: we cannot know.

We do know, however, that the threat already exists and is getting more baleful.  Roughly 700 Britons have travelled to Syria to fight for ISIS.  Over half these, about 450, have returned.  118 of them are being monitored by police.  There were 165 Syria-related arrests in 2013, up from 25 during 2012.  But the menace doesn’t come only from British Muslims who have come back from fighting abroad.  Almost 300 people were arrested in the year up to last March on terror grounds – an increase of almost a third on the previous year (about a third of those arrested were charged).  This is a record number.  Arrests are now taking place at a rate of more than one a day.

So although air strikes on ISIS in Syria are indeed likely to raise the likelihood of terror attacks here, the chances of these being carried out are already increasing – if you believe, as reason suggests, that the more plots are hatched, the more likely one is to succeed.  Each time one is foiled, ISIS, or Al Qaeda, other Islamist terror groups and lone wolves could, perfectly appropriately, join together to repeat the words of the IRA after the Brighton Bomb that failed to murder Margaret Thatcher: “Today we were unlucky, but remember we only have to be lucky once.”  Or twice.  Or many.

Furthermore, foreign policy is not the only or even the main reason for Islamist terror.  Some of it is motivated more by foreign affairs than policy: the first known British suicide bomber was Asif Sadiq, a 24-year-old student from Birmingham.  He died not in Britain, or even in the Middle East during the Iraq War, but in Indian-administered Kashmir, as long ago as 2000, driving a stolen car packed with explosives into an army base.  The Kashmir issue is a very live one among British Pakistanis and Kashmiris, but we have had nothing much to do with it since 1947 – the best part of 70 years ago.  Sadiq’s terror assault is a reminder that the link between political issues abroad and Islamist terror here reaches far wider and goes far deeper than Syria’s civil war.

The Muslim-majority world has fallen behind the West, in terms of economic and social power and projection, since roughly the time of the renaissance.  After 1945, the vogue means in the Middle East and elsewhere to lever that world back was authoritarian secularism.  Bashir al-Assad is a survivor of that tradition.  His father was a Baathist – the follower of a form of Arab socialist ideology.  Saddam Hussein was one, too.  Muammar Gaddafi swam in the same stream of ideas.  But as this form of secularism failed to rescue the Muslim-majority world (and was seen to do so) and Islamist extremism has gradually emerged as a replacement.  Wahaabi beliefs and the western idea of the totalitarian state have merged to create Islamist doctrine, of which ISIS is only the most extreme practitioner.

That ideology does not hate liberal democracies because of their foreign policy only, or even mainly.  It hates them not for doing but for being.  We are understandably gripped by the latest horror in Paris, but we should remember the last one.  The Charlie Hebdo killings had nothing to do with foreign policy, or even foreign affairs.  They were driven by a clash between the modern world of free expression and the pre-modern one for which Islamists yearn, in which free thought and action is stifled by the state.  That latter world already has a grip on this one – as we see in, say, Pakistan, with its oppression of Christians and other religions minorities, and as we have seen here since the time of the Satanic Verses row.

So when MPs vote on extending the British bombing of ISIS that is already taking place in Iraq to Syria, they obviously have a responsibility to ask what the effect of air strikes abroad (and other action) will have here.  But they also have one to ask what happen if that action is not taken – and, more broadly, to grasp and explain that foreign policy is not the sole or main contributor to Islamist extremism and Islamist violence.  The Strategic Defence and Security Review about which the Prime Minister writes in today’s Daily Telegraph is part of a gradual of Britain’s collective defence focus from largely abroad to increasingly here. So whether or not bombs in Syria increases the threat of terror here turns out not to be the only question about such action – or even the main one.  That, rather, is whether or not air strikes would be effective.  We will return to that question later this week.