A SHIVER of apprehension ran down my spine when I heard that Downing Street has said that the UK is drawing up contingency plans for military action in response to the chemical weapons attack in Syria. And the Foreign Secretary William Hague said that unilateral military action might be needed without the sanction of the United Nations.

We have been here before, as we were reminded when Tony Blair joined the debate this week, saying that the enduring controversy over his decision to invade Iraq in 2003 should not stop politicians from acting now on Syria. We should stop wringing our hands, he said. But military action is not the only alternative to hand-wringing. It is by no means clear that all other means are exhausted.

In the United States, President Obama painted himself into a corner last year by saying that the use of chemical weapons would be a "red line" which, if crossed, would force US intervention.

Now they have indisputably been used. Yet, although the West seems to be of one mind that the Assad regime is responsible, we have not yet seen the proof, and that matters. Chemical weapons, Washington said this week, were a "moral obscenity". Some weapons are certainly more horrific than others. Chemical weapons raise our levels of disgust and outrage. But it is not clear that they alter the moral argument.

An application of the precepts of the just war shows this. Intervening to prevent the killing of children and other innocents is clearly a just cause. Right intention is evident, too. But the just-war criteria demand that force is a last resort, has competent authority, is proportionate, and has a good prospect of success. None of these are fully present.

Although the international diplomatic situation is fixed, it is not yet a stalemate. Russia has made movement in agreeing to force Syria to allow UN weapons inspectors to the site where 300 or more died last week. Competent authority would suggest a resolution by the UN Security Council, on which Syria's allies Russia and China have a veto. The requirement is not absolute; Kosovo was a just intervention without a UN resolution, but Iraq showed how problems can arise from precipitate action.

Proportion sounds plausible, with the talk of a single air strike to signal to President Assad that he cannot use chemical weapons with impunity. But history suggests that air strikes have a tendency to escalate. In the past, they have led to Western boots on the ground, and situations from which exit strategies become tricky to devise, and which radicalise the opposition. But escalation could also spread the conflict to Lebanon, Turkey, Iraq, Israel, and even Iran. Where would arguments about proportionality stand then?

The "prospect of success" is deeply problematic, too, for it is not even clear what would constitute success. The Shadow Foreign Secretary, Douglas Alexander, showed some insight on this, when he asked whether military action would or could simply "degrade Assad's chemical-weapons capability", or whether it would involve taking sides in a bitter civil war. Since al-Qaeda is prominent in the rebel alliance, this is not a simple matter.

The outraged demand that "something must be done" should not bully us into doing the wrong thing. A signal needs to be sent to President Assad that he cannot use chemical weapons with impunity. But it could yet be diplomatic. Russia and Iran were both pressured to shift on their intransigence against UN weapons inspections. That has shown that the international disunity on which the Assad regime has relied need not be permanent. There is more to be achieved by diplomacy before the Cruise missiles are dispatched.