In the early hours of 21 August 2013, rockets began to land in the Damascus suburb of Ghouta. The civilian population of Syria had now become used to this, since Bashar al-Assad had decided over 2 years earlier that in response to a peaceful uprising against his totalitarian rule he would prosecute the most brutal military campaign by a ruler against his people that this century has seen. But this attack was different: the rockets were filled with sarin, a highly toxic nerve agent.

When the images of the hundreds of people killed and thousands injured began to circulate, there was international outrage of a level not so far seen in the Syrian Civil War. Momentum gathered for a military response. Obama’s red line had been crossed. Enough was enough.

Only it wasn’t. Obama dithered. Miliband played politics. Assad survived to kill another day.

And the war rumbled on. Assad continued brutalising. Barrel bombs continued to fall on schools and hospitals. And, of course, a new force took hold in the vacuum that northern Syria had become: jihadis from around the region, and later the world, gathered and declared the creation of their caliphate.

The use of chemical weapons by a state, not against enemy combatants but directed deliberately against civilians, is the sort of clear and unimpeachable example of a war crime as it is possible to imagine, and overwhelmingly justified (and indeed necessitated) a military response. The lack of such a response has not only prolonged the war in Syria, it has turned the use of chemical weapons from an action likely to be met by overwhelming force from the international community to a risk-free tactic for every megalomaniac with the capacity to inflict such a sin.

The case for UK airstrikes against Islamic State in Syria is not such an overwhelming one. Not that IS has not committed war crimes of unspeakable horror: it has, and is. And IS is more of a direct threat to the citizens of Europe and the US than Assad, so the proximity of its barbarism is a factor.

Nobody, though, expects airstrikes, and specifically Britain’s participation in them in Syria, to ultimately defeat IS. Unlike Assad, the coalition of forces (with the exception of Russia) engaged in air strikes will always, inevitably, seek to minimise civilian casualties. In those circumstances, airstrikes can only ever be part of a solution to take out militants where they are living amongst civilian communities.

And whilst there are regional ground troops with whom airstrikes are coordinated, they have not proved themselves capable of resounding victory over IS (though there has been some success in Iraq).

The case for UK airstrikes

There seems to be a consensus among Liberal Democrats that Islamic State needs to be destroyed, and that the UK should play some role in that given the direct threat if nothing else. The primary question therefore is what that action should be.

Hopefully there is also a recognition too that IS will only be defeated militarily. They cannot be negotiated with; they are ultimately a military organisation and will only be defeated militarily. It seems to me that the logic from that point may not have been followed through, though.

Once one has accepted that IS needs to be defeated, that we should play a role in that defeat, and that force is needed, there a few choices available, and none of them is particularly palatable.

One would be a broad a coalition as possible putting troops on the ground, including British forces. That would almost certainly lead to a defeat for IS in a relatively short period of time. But if is also politically impossible.

Broadly the other option is for local actors to do the ground fighting and those nations who can to support that from the air and in other ways (with intelligence and equipment) – essentially the current proposal.

That is less likely to guarantee ultimate success, though there are signs that it is a strategy that is working in Iraq, where IS has lost territory.

The other question, which the prime minister has referred to, is whether any action we propose should only take place after a wider negotiated solution is reached to end the Syrian civil war, or whether we should act against IS now.

Despite some progress in the Vienna talks, the Russians and Iranians still seem to be strongly protecting Assad, whose departure is a pre-requisite to any negotiated solution, both because of the impossibility of him fronting a peaceful settlement in a united Syria (given the tens of thousands of civilians he has killed) and because of his responsibility, direct and indirect, for the rise of IS in the first place, and for its continued success.

Given the remoteness of a negotiated solution at present, therefore, the case to delay action is essentially a case for no action, and IS will continue to maintain its strength in Syria at least, inflicting horrific brutality on people in the regions under its control, recruiting more and more jihadis from around the world, and exporting its terror to Paris and elsewhere.

So whilst a negotiated settlement to the wider conflict is fundamental to guaranteeing IS’s ultimate destruction, the action in Iraq has shown that they can at least be significantly degraded without western troops on the ground, through coordination between western air forces and local troops. Weakening IS in preparation for an ultimate defeat as part of a wider settlement is not the ideal scenario, but it is certainly better than nothing.

The final argument is that given the US and France (and, when they are not assisting Assad, Russia) are already all bombing IS in Syria, UK assistance would add little. However, that ignores completely the advice of the military figures who ultimately know whether or not that is true, and they say that whilst British involvement will not be significant in terms of the number of sorties, the RAF has technical abilities that are not shared by the US and France and can therefore make a valuable contribution, particularly in ensuring strikes are as targeted and precise as possible.

Decisions on whether to take military action are rarely clear-cut. The case for UK airstrikes against IS in Syria is not an overwhelming one. There are some good arguments against. There is no guarantee of success, even on the limited terms we set. But the case is sufficiently strong that Liberal Democrat MPs should back the government’s motion when it comes up for a vote.

* Nick Thornsby is a day editor at Lib Dem Voice.