The Syrian regime’s use of chemical weapons flouts the rules of war and must be punished. President Donald Trump’s press secretary, Sean Spicer, this week warned that “if you gas a baby, if you put a barrel bomb into innocent people…you will see a response from this president.”

Meanwhile Rex Tillerson, the American Secretary of State, laid a wreath in the Tuscan village of Sant’Anna di Stazzema, where the Nazis murdered 500 people, and declared: “We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world.”

Such sweeping language could be used to justify an extraordinarily wide range of interventions. But it also justifies the attempt, however inadequate, to write a profile of the rules of war.

One should not of course imply that the internationally agreed laws and conventions which are supposed to govern the use of armed force necessarily do so.

As one British expert commented, when I told him I had been asked to write a profile of the rules of war: “People will generally do whatever they think they can get away with.”

But what you can get away with in a democracy is conditioned not just by whether people think what you are doing is moral and practical, but whether they think it is legal.

One has only to recall the lengths to which Tony Blair went to make the legal case for the Iraq War, and the corresponding energy with which his opponents sought to show he was acting illegally, to see that the question matters more than a purely realist conception of foreign policy might suggest. Law, it turns out, is one of the many realities which has to be taken into account.

At the weekend, Sir Michael Fallon, the Defence Secretary, took great care, in a piece for the Sunday Times, to indicate the legal basis for the American action:

“After the horrors of chemical attacks in the Great War, the use of asphyxiating, poisonous or other gases was banned by the Geneva Protocol in 1925. However, President Bashar al-Assad’s regime has a history of using such weapons against its people. It flouted international law in 2013 by gassing nearly 1,500 people to death outside Damascus.

“Yet despite agreeing later that year to destroy its chemical weapons, Syria has again been caught in the act. Airstrikes are never undertaken lightly but this is a wholly unacceptable situation. Something had to be done to stop more people dying.

“So, in his first test as commander-in-chief, President Trump made the right call by resorting to careful and narrowly focused military action.”

What is the difference between a gas attack and a barrel bomb? According to the Syrian Network for Human Rights, the Assad regime last year dropped 13,000 barrel bombs. It stands to reason that these must have inflicted vastly greater suffering on innocent men, women and children than the most recent gas attack has done.

In moral terms, as Spicer suggests, to kill someone with either is abhorrent. But in legal terms, as Fallon indicates, there is a significant difference. So the White House had to correct what Spicer had said about barrel bombs: a prelude to the correction and apology he himself had to make a day or two later after his deplorable claim that Hitler did not use chemical weapons.

The Hague Conference of 1899, which drew up the first of the Hague Conventions, made various attempts to lay down the laws and customs of war. It included declarations “on the launching of projectiles and explosives from balloons”, “on the use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases”. and “on the use of bullets which expand or flatten easily in the human body”.

Gas was nevertheless employed on an extensive scale during the First World War. A renewed attempt to outlaw it was therefore made in 1925, in the Geneva Protocol on Asphyxiating or Poisonous Gases. In 1968, Syria acceded to this protocol.

The Geneva Convention of 1949, which was ratified by Syria in 1953, and has indeed been ratified by every single member of the United Nations, extended protection to civilians: previous conventions had applied only to combatants.

Other weapons which have been treated as particularly noxious include landmines, laser weapons, biological weapons and cluster munitions. But neither Syria nor some other countries have ratified all these later protocols.

To suggest that these agreements have had no effect would be as unrealistic as to pretend they have never been broken. Some amelioration of the horrors of war has sometimes been achieved.

The 1997 Chemical Weapons Convention, administered by a nongovernmental organisation based in the Hague, is a treaty prohibiting the development, production, stockpiling and use of chemical weapons. Syria not only refused to sign it, but stated in 2012 that it possessed chemical and biological weapons and would use them if attacked.

On 21 August 2013 the Ghouta chemical attack took place on two opposition-held suburbs of Damascus. On 29 August, Britain’s Joint Intelligence Committee said at least 350 people had been killed, and it was “highly likely” that the attack had been carried out by the Syrian government, which had used chemical weapons on a smaller scale on 14 previous occasions.

On the same day, the Commons debated whether to take part in military action in response. The then Prime Minister, David Cameron, said:

“The question before the House today is how to respond to one of the most abhorrent uses of chemical weapons in a century, which has slaughtered innocent men, women and children in Syria. It is not about taking sides in the Syrian conflict, it is not about invading, it is not about regime change, and it is not even about working more closely with the opposition; it is about the large-scale use of chemical weapons and our response to a war crime—nothing else.

“Let me set out what the House has in front of it today in respect of how we reached our conclusions. We have a summary of the Government’s legal position, which makes it explicit that military action would have a clear legal basis.”

MPs nevertheless voted by 285 to 272 against taking military action. Ed Miliband, the Leader of the Opposition, said after the debate that people didn’t want “a rush to war” and Britain “doesn’t need reckless and impulsive leadership, it needs calm and measured leadership”.

But re-reading Cameron’s speech now, one can see him going out of his way to provide calm and measured leadership, and to demonstrate the legality of what he proposed:

“We must ensure that any action, if it is to be taken, is proportionate, legal and specifically designed to deter the use of chemical weapons.”

Barack Obama nevertheless decided the United States would refrain from taking military action. He instead struck a deal, brokered by the Russians, under which the Assad regime agreed to to give up “every single bit” of its chemical weapons and to join the Chemical Weapons Convention.

It now appears overwhelmingly likely that the Assad regime broke the agreement and kept back the sarin which was used in the latest attack. The New York Times remarked a few days ago that this shows the danger of “doing deals with dictators”.

One might also say it shows the limitations of a measure like the Chemical Weapons Convention. Three states, North Korea, South Sudan and Egypt, have neither signed nor acceded to that treaty, and one must reckon there are others which, like Syria, flout its provisions.

But amelioration rather than total prevention was the original aim of the Geneva Conventions. In 1859, a Swiss businessman, Henri Dunant, happened to witness the aftermath of the battle of Solferino, fought between the French under Napoleon III and the Sardinians under Victor Emmanuel II against the Austrians under Franz Joseph.

Dunant was appalled by the wretched inadequacy of the arrangements for looking after the many thousands of casualties scattered in agony across the battlefield. He did what he could to organise local relief efforts, given irrespective of the national origins of the wounded, and wrote a book, A Memory of Solferino, which a few years later led to the setting up in Geneva of what became the International Committee of the Red Cross.

Rules of war have always existed in various customary forms, and from the 17th century onwards acquired parliamentary authority as applied to the Royal Navy and the British Army. According to the American Law of War Manual issued in 2015 by the Department of Defense,

“The law of war is part of who we are. George Washington, as Commander in Chief of the Continental Army, agreed with his British adversary that the Revolutionary War would be ‘carried on agreeable to the rules which humanity formed’ and ‘to prevent or punish every breach of the rules of war within the sphere of our respective commands’.”

But international humanitarian law sprang from the Red Cross, with the first Geneva Convention signed by 16 countries in 1864. The Geneva Conventions seek in time of war to uphold the protection of prisoners and of civilians; provide for the care of the wounded; ban torture; and prohibit methods of warfare which cause excessive suffering.

The more seriously one takes international law, the more sceptical one may be about President Trump’s willingness to observe it. Professor Philippe Sands, of University College London and Matrix Chambers, who appears frequently before international courts and whose most recent book, East West Street, explores the origins of genocide and crimes against humanity, declared in 2015 that “We need a Syria strategy, not half-baked reasons to drop bombs”, and reacted with horror when I asked him about the most recent American attack:

“Is there a plan? Is there a policy? Is it legal? Decisions taken on the hoof invariably risk escalation as one thing leads to another. Applause on Syria and chemicals, why not North Korea and nukes?”

And yet as someone who has always felt instinctively sceptical about international law, I cannot help feeling impressed by the way it hangs together, and provides a basis for distinguishing between civilisation and barbarism.

It cannot provide a substitute for statecraft, or for the exercise of wider judgment about whether a particular war is just. For that, one needs Thomas Aquinas, recently examined by Charles Guthrie and Michael Quinlan.

A former officer in the British Army observed that international law does not restrain bands of irregulars, whether in the Balkans or Ukraine or Syria, where a bunch of thugs may operate under the command of the biggest thug.

But that same officer spoke movingly about the horror of gas as a weapon. If the Chemical Weapons Convention did not exist, it would have to be invented. And who can honestly maintain that it ought not to be enforced?