A hundred experts from the Organisation for the Prohibition of Chemical Weapons, which on Friday won the Nobel Peace Prize, and the United Nations are assembling to dismantle and destroy Syria's arsenal of chemical weapons including 1,000 tons of sarin and mustard gas as well as chemical mixing equipment. All are to be eliminated by 30 June 2014.

It is one of the extraordinary twists and turns of the war in Syria that the alleged use of sarin against civilians in rebel-held districts in Damascus on 21 August should turn out to be to the advantage rather than to the disadvantage of President Bashar al-Assad. The most immediate effect seemed likely to be foreign military intervention against Assad. In the event, the United States and Britain balked at the idea of another war in the Middle East, particularly one that might put in positions of power al-Qa'ida-linked groups such as the al-Nusra Front and the Islamic State of Iraq and the Levant .

States acquire chemical weapons in order to frighten or kill but, like the man in the fairy story who owns a bottle with a powerful genie inside, they often end up getting the opposite of what they wish for. Saddam Hussein used poison gas with typical ruthlessness against the Iranians and the Kurds, and may have congratulated himself on its effectiveness. But in the long term he found that possession of weapons of mass destruction, and his inability to prove that he had destroyed them, provided justification for ruinous UN sanctions against Iraq and the US-led invasion of 2003.

Download the new Independent Premium app Sharing the full story, not just the headlines

Chemical weapons are often described as "weapons of last resort", but a country that is already losing a war cannot use them without inviting calamitous retaliation by those about to win. This appears to be why Hitler did not use Germany's massive arsenal of chemical weapons when his armies were going down to defeat in 1943-45. The Germans also wrongly believed that the Western allies had developed nerve gases such as sarin and tabun, discovered in Germany in the late 1930s.

In fact, it was Britain that came closest to using poison gas on a mass scale during the Second World War. Had Germany launched an invasion in 1940, the British plan was to spray German troops with mustard gas from aircraft while they were still crowded together on the beaches. The idea was proposed first on 15 June 1940, just two days after Dunkirk, by the Chief of the Imperial General Staff Sir John Dill according to A Higher Form of Killing: The Secret Story of Chemical and Biological Warfare by Robert Harris and Jeremy Paxman. General Dill wrote with gusto that German troop concentrations "would present a splendid target. Gas spray by aircraft under such conditions would be likely to have a more widespread and wholesale effect than high explosives."

The plan was lethally dangerous, not least because Germany had 20 times as much mustard gas as Britain. The British would be breaching a secret agreement made with Hitler in the first hours of the war in 1939 that Britain and France would not use poison gas or germ warfare so long as Germany also refrained. The appalled Director of Home Defence rejected the plan to spray enemy troops: "We should be throwing away the incalculable moral advantage of keeping our pledges and for a minor tactical surprise; and the ultimate effects of retaliation by the enemy would be very serious in this overcrowded little island."

General Dill withdrew his memorandum in the face of fierce criticism, but it was then supported by Churchill who gave his full backing for the use of gas. A fleet of bombers fitted with spray tanks holding between 250lb and 1,000lb of mustard gas each was scraped together but Britain had only 450 tons of mustard gas. Stocks would have been exhausted after one or two days of RAF attacks.

Even in the desperate situation after Dunkirk, the plan seemed foolhardy. Spraying troops from a low level in the face of hostile air attack would have been very difficult, and regular troops with gas masks and other anti-gas equipment would not necessarily have been incapacitated. The English coast where the German army intended to land was between Folkestone and Newhaven on the south coast, a parachute division dropping near Folkestone. The main beachheads would have been the wide pebble shores on either side of Dungeness and the flat land of Romney Marsh.

Would Churchill really have ordered the use of poison gas when supplies of it were so meagre and German retaliation against London likely to be massive and immediate?

Churchill's determination to use it in the case of German invasion did not ebb, even as the chances of an attack receded. Because of his efforts, Britain had 20,000 tons of poison gas ready to use by 1942. Conventional wisdom and military war games suggest that a German invasion force could have got ashore but would have been cut off by the Royal Navy and, ultimately, wiped out. The Navy, in turn, could not have been eliminated without total German air superiority, which it failed to win in the Battle of Britain. This was most likely the case, but German defeat was not quite so certain in 1940.

I have always been fascinated by the non-use of poison gas in the Second World War, because it showed restraint even in a time of total war. Most probably its use against London and other British cities would have provoked mass flight. In 2003, the Kurds in northern Iraq, who had seen what poison gas could do, were convinced that Saddam still had stocks of it. I remember shops selling flimsy plastic sheeting for people to put over windows, which only advertised citizens' vulnerability. Most Kurds fled their cities.

Exotic terror weapons such as gas inspire revulsion in a way conventional weapons do not, though high explosive shells and bombs are as lethal. People from Eastern Ghouta in Damascus complain that nobody has objected strongly to the fact that they have been shelled and bombed. In considering what happened in Fallujah, Iraq in 2004, people are obsessed with the possible use of depleted uranium, but take it as a matter of course that the US Marines' artillery pumped a daily average of 379 155mm shells for two weeks into this not very large city. One evil effect of chemical weapons is making conventional weapons that kill people in their hundreds of thousands seem acceptable by comparison.