American and British bombing, along with that of their allies, degrades ISIS in Iraq by disrupting its control structure and destroying its key assets – just as NATO bombing did in Yugoslavia in 1999 (according to one version of history, at least).  Civilian casualties are minimised.  This success is followed by the defeat of ISIS and the end of “Islamic State” on the ground, as the western-backed Kurds and Iraqi army regain lost territory.  Purged of Nouri al-Maliki, Iraq swiftly finds its way to a settlement acceptable to Shi’ites, Sunnis and Kurds alike, perhaps by the means of the federal solution outlined on this site by our columnist, Nadhim Zahawi.  Public backing in Britain for military action stays high, ensuring that Labour and Liberal Democrats stay on-side with the Conservatives.

The United States and those allies act similarly in Syria, with Britain joining them at a later date, and a revived Syrian Free Army both sees off the extremist Islamist groups and fights the Assad regime to a standstill.  The interests of America, Russia and Iran converge in the removal of Assad, and his replacement by a Baathist-aligned Alawite general who seeks to work with the Sunni majority in the country.  The Sykes-Picot borders are re-established.  Everything settles down.

This is one version of what could happen if the Government Britain joins the American-led military action.  Here is another.

The bombing indeed disrupts ISIS’s control structure and degrades its assets, but finds it hard to identify and eliminate its key personnel.  Civilian casualties are high. Israel, which is used to targeting Hamas operatives, none the less killed civilians in its recent Gaza operation, as previously.  The allies have less experience of targeting terrorists, and civilian casualties are thus higher.  This is followed by the more radicalisation towards extremism in Britain, more British Muslims finding their way to Iraq and Syria – and terror attacks here.  The Iraq army remains weak and its government corrupt. The American-led alliance gradually breaks up.  Qataris and Saudi Arabians continue to help fund ISIS. Support for it hardens in the territory it controls. The so-called “Caliphate” digs in.

Meanwhile, Assad gains the upper hand in parts of Syria, but its long civil war stretches on into its fourth year.  The Syrian Free Army makes no breakthrough.  Public backing in Britain for military action falls but doesn’t collapse – since none of the main political parties favour sending ground troops to Iraq (though some special forces are used), and the Commons, which has seized control of foreign policy from the Government, would not vote for their deployment in any event.

You must judge for yourself which of these outcomes is more likely.  For what it’s worth, my view is that the second scenario is closer to what will happen, though another possibility lies in between.  Islamist terror has been defeated in Iraq before – and recently.  Al Qaeda first flourished and then failed there between 2004 and the end of the Bush “surge”.  Some argue that it was the sheer number of American troops that made the difference; others that it was special forces and operations.

But either way, what mattered at least as much as American forces was Iraqi civilians.  Al Qaeda posed as the champion of Sunni Iraqis.  But its terror turned most of them against it: they soon saw that it was fixated on its barbaric vision, not their welfare.  The lesson here is a commonplace: others can help, but only Iraqis can create a stable Iraq.  It should be Iraqi and Arab airplanes that will carry the air war to ISIS – not British and American ones.  However, this won’t happen.

The Commons should ask David Cameron some hard questions tomorrow, all the more so because the leadership of the other two main parties has apparently been squared.  MPs should do so not because there is a danger, if British bombing takes place, of a new, heavy and tragic flow of military coffins to Brize Norton, but because they should strive, as always, to probe the chances of success and get to the heart of the matter.

Here are some lines of enquiry. How likely is British bombing of Iraq to achieve anything – other than to make us feel that we are somehow hitting back against “Jihadi John”?  Is ISIS deliberately provoking  western-led bombings, in order to establish itself as the champion of the “Arab Street”, and if so are we flying into a trap?  Isn’t bombing more likely to work if it has the support of special forces on the ground, working in conjunction with local forces, even if the mass use of ground troops is (rightly) ruled out?

Before the original invasion of Iraq, the Pentagon was convinced that, as locals greeted American troops, “flowers would be stuck on the end of rifles”, according to Admiral Lord Boyce.  Ambitions since have grown more modest.  When British planes fly to Iraq, as they surely soon will, will they really be doing more than “mowing the grass”?