The outline of a solution to Syria’s civil war is easy to describe but would be very hard to implement.  The United States and Russia would join to lead a diplomatic and political settlement, enforced by troops on the ground, in which a transition took place from Bashar al-Assad’s regime to democratic elections.  The United States would lean on Saudi Arabia, from which support for ISIS has come; Russia would lean on Iran, Assad’s main supporter in the region.  Muslim-majority countries would provide the ground forces.  Other nations would join them in carrying out air strikes on ISIS.  The plan would be backed by a U.N Security Council Resolution.

The harsh truth to date, however, is that too many of the players in this drama have had interests more important to them than destroying ISIS’s force’s on the ground – and that some, such as Saudi Arabia, are ambiguous about the terror group in the first place.  Russia’s main interest is in propping up Assad, not dealing with ISIS.  Barack Obama is suspicious of entanglement abroad.  To Assad, ISIS offers an opportunity as well as a threat – namely, to present his regime internationally as the only workable alternative to it (which is why he and Russia have concentrated their fire, to date, on other Islamist groups and on pro-western forces).  ISIS is useful to others, too.  Iran paints it as the logical end-point of Wahabi fundamentalism, thereby pursuing its own drive for international rehabilitation.  Some in the Iraqi Government, which itself is Shia-dominated, see matters the same way, and are ambiguous about replacing ISIS in the Sunni-majority parts of Iraq that it dominates.  Turkey is preoccupied with what it sees as the threat of a Kurdistan carved partly out of its territory, and is thus not lined up behind the Kurds’ own battle against ISIS.

This babel of aims, made even more convoluted by Russia’s own recent military intervention in Syria, made British air strikes against ISIS look more remote than ever until last weekend.  The Paris horror, and more directly the Sharm El-Sheikh atrocity, has shifted Vladimir Putin.  He now seems to be persuaded of the case for attacking ISIS rather than simply backing Assad.  Russia has stepped up air strikes against the terrorists.  There is a U.N Security Council Resolution supporting the use against ISIS of “all necessary measures”.

This is David Cameron’s moment.  Today’s papers report that he is pushing for a Commons vote soon, arguing that Putin’s change of outlook means a united front against ISIS, that the political and diplomatic settlement I describe above is therefore now possible – with Russia forcing Assad out of government – and that British air strikes are a useful military complement to it since, as the Sunday Telegraph puts it, “the RAF’s high-precision Brimstone missiles are required for more accurate targeting of terrorist vehicles”. The Prime Minister’s loathing of radical Islamism is not in doubt.  He has worked hard to put in place a policy, here in Britain, that targets extremist ideology as well as actions.  But there is also a domestic political angle.  A Parliamentary vote on air strikes would split Labour – apparently at front bench level, too – and make it look even less like a potential party of government than it does already.  And there is a personal consideration as well as a political one.  Cameron felt his Commons humiliation over the 2013 vote on Syria very keenly, and will not have enjoyed looking weak in the eyes of Washington.

We argued during the summer that it makes no sense to stop bombing at a border which no longer exists.  If Russia is on board for a united push against ISIS, the case for it becomes compelling again.  Furthermore, Conservative MPs will recognise that the importance of British air strikes would be at least as much symbolic as military – a clear sign that Britain stands united with France – and that these would not commit us to sending in ground forces, Iraq War-style.

It would be pusillanimous not to join France in Syria for fear of an ISIS attack here.  After all, Islamist terror groups are targetting Britain already. But Tory backbenchers should go into any vote with their eyes wide open (and Downing Street should be sure that their backing fis solid before any division takes place).  It is far from clear that a solution to Syria’s terrible civil war is really any nearer post-Paris than it was before.  Putin’s priorities and plans may change and, in any event, there is little sign that he is willing to throw over Assad.  Above all, our own front line against Islamist terror is neither there nor in Iraq but here, at home in Britain.  In the long struggle against it in Europe, informers and intelligence will count for more than air strikes and Brimstone missiles.