Two summers ago, the Commons voted against David Cameron’s proposal to bomb the Assad regime.  This time round, he wants it to support bombing ISIS in Syria: ‘We’ve already carried out more airstrikes in Iraq than anyone else, other than the US,’ he said yesterday. ‘But I want us to step up and do what I call a full-spectrum response. That means hammering IS in Iraq and helping with what the US is doing in Syria.’

That the original target of British bombs was to be the Assad dictatorship and that it is now its terrorist opponent – in Iraq and, if the Prime Minister gets his way, in Syria – says much about the inconstancy and confusion of Government aims amidst the civil war within Islam.  On the one side is Assad, backed and funded by Iran.  On the other is ISIS, supported – up to a point – by Saudi Arabia and Qatar.

ConservativeHome opposed Cameron’s original proposal, on the threefold ground that bombing might not deter Assad from using chemical weapons (the Government clearly had no Plan B were he to have continued to do so); that Britain should be not be sucked into Syria’s civil war, and that the main terror threat to our own citizens comes from extremist Sunnis, not extremist Shiites.

This being so, there is now a strong case for widening the air strikes against ISIS in which we already take part.  As the horror in Tunisia proved, both the terror group and its message is a threat to British citizens.  The border between Iraq and Syria is now real only on maps.  It therefore makes no sense to stop attacking ISIS’s forces when they flee to the latter side of it.

Furthermore, such action would be unlikely to draw Britain into Syria’s bloody civil war, since it would not come – as the proposal to bomb Assad regime targets did – with an ultimatum.  This fact, however, is a reminder of its limitations.  Bombing can disrupt ISIS, but it cannot defeat it.  For that, one needs “boots on the ground”.

In this regard, there is a role for special forces, which helped to defeat Al Qaeda in Iraq during the latter days of the Bush presidency.  But there is no appetite in Britain or America for the mass deployment of troops.  Voters have rightly concluded from the Iraq War experience that the sacrifice of blood and treasure which would be required to bring order would be too great, and wouldn’t work in any event.

The Prime Minister was stung by Ed Miliband’s about-turn in 2013 – itself a sign of how deeply Labour has been traumatised by Iraq.  Harriet Harman is more open to air strikes in Syria, which is why he is gingerly exploring the proposal.  But her authority runs shallow in the party she temporarily leads, as its recent revolt over the Government’s welfare proposals indicates.

Labour will have a new leader this autumn, and Cameron will surely want to get his or her measure before putting any plan to Parliament.  But it will be even more important to take his own party with him.  He cannot carry on where he left off over EU referendum purdah, and rely on Labour votes in the lobbies (or abstentions) in order to outvote anxious Tories – or he should not, at any rate.