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As British Foreign Secretary in 2013, faced with the use of chemical weapons in Syria, I recommended that Britain join in missile strikes against the Bashar Al Assad regime. The House of Commons refused to agree to do so, opening the way for emboldened Russian intervention and further such atrocities in the future.

One of those has just happened, as the dead bodies of children with white foam in their mouths, utterly innocent victims of a vile dictatorship, demonstrate all too vividly. If I were still in office I have little doubt what I would recommend today.

The United States should take swift military action, more extensively than last year when similar crimes took place, against the military facilities of the Al Assad regime. The UK and France should join in if they can. Sufficient notice of such action should be given to Russia to allow for lives, but not necessarily equipment, to be saved. Crucially, the US should announce that it will not be withdrawing from Syria the forces that have successfully spearheaded the fight against the Daesh( self-proclaimed Islamic State in Iraq and the Levant) terrorists.

Clear though it is to me that this should be done, the searing experience of 2013 reminds me that many other people in Britain might not see things that way. There are four main objections raised to mounting any retaliation for chemical weapons strikes in Syria. One is that any action should have the authority of the UN and unilateral moves should be avoided. This is the argument in which Jeremy Corbyn will take refuge.

The answer to this is that the Russians use the UN to obstruct, deny and prevaricate. The Kremlin does not care whether Al Assad uses chemicals, torture, sexual violence or any other method so long as he wins. The UN Security Council is the forum in which to confront Russia and Syria over this, but not the means of redress. A second objection, much in evidence in 2013, is that somebody else might have done it. Why would the Syrian regime do this, it is asked, when it is winning anyway? Perhaps it is being framed. This is on a par with the naivete of those who question why Russia would have tried to kill Sergei and Yulia Skripal. Brutal systems and the people who control them get used to killing. Depressingly, they often get away with it, but sometimes they miscalculate and get caught.

For Al Assad’s regime, this is just one atrocity among hundreds. And just as only one country produces the nerve agent that was used on the Skripals, so it is only the regime that possesses and uses chemical weapons in Syria.

A third objection is that this is indeed only one of many ways of killing large numbers of people. Why get so excited over chemical attacks when so many have died from bombs and bullets? Not many people say this, but it is what they think. Reply to this is that the world has succeeded for nearly a century in preventing the use of chemical weapons on the battlefield. Once we accept that it is just another aspect of war, that is what it will become in the conflicts of coming decades, with an arms race in chemical agents steadily expanded and legitimised.

The final and most substantial objection is that retaliation will only make matters worse, and in the case of Syria today risk a wider war between outside powers. Yet had the US and UK taken action in 2013, there would have been a far better chance that subsequent outrages would have been discouraged.

It is easy to criticise Western interventions in Iraq, Libya or Afghanistan, but non-intervention can also carry a terrible price. The humiliating refusal by the Obama administration to enforce the “red line” of which the president had spoken, I and other Western ministers continued to attend innumerable meetings to try to resolve the Syrian conflict. But we were left with only words, and compared to other nations financing armies or sending forces, words count for very little. We had become enfeebled spectators of one of the most destructive conflagrations of our time. Our efforts to bring about peace evaporated without a readiness to use hard power when it was needed.

Since then, Western armed forces, including our own, have re-established some of our credibility by working with local forces to crush an emerging terrorist state in Syria and Iraq. That successful effort, along with a long war drawing in other countries, means there is now a dangerous combination of Russian, Turkish, Kurdish, Israeli, Iranian and American forces in proximity.

That is why, when President Donald Trump orders US naval forces to unleash missile strikes at Syrian military targets — which is his most likely choice of action — very precise calculations of what to hit are necessary. His evident instinct to act against chemical weapons attacks is the right one. But his reported recent decision to withdraw the remaining US forces from Syria since Daesh is apparently defeated is a mistake.

Dangerous though the jostling of outside forces is, leaving it to authoritarian rulers in Moscow, Ankara and Tehran does not make it less so. A full US withdrawal from the ground could easily be followed by the revival of the terrorist threat, or by a crisis over the Kurds, or by Israel being drawn deeper into a continuing conflict. Washington will find it needs the leverage of being still present in Syria, however popular it has become to get out fast.

So if Trump launches serious retaliation he will be fully justified, and we should back him any way we can. But if he tells his military chiefs to stay in Syria he can also have a wider influence on events. We should have learnt from the fiasco of 2013 that abdication of the responsibility and right to act doesn’t make war go away.

— The Telegraph Group Limited, London 2018

William Hague is a former British foreign secretary.