Over the last two years more than 110,000 people have been violently killed just across the border in Syria. That’s approximately 150 a day, or six an hour. One person killed every ten minutes, for two years. And the world has done nothing. More shocking, at least at first glance, is that very few people have called seriously for intervention.

The terrible truth is that there is nothing the West can do. There simply are no good options. Over time the conflict has become more and more sectarian, pitting the Alawite ruling class against the Sunni majority. The rebels became more and more radical, as groups like Jabhat al Nusra gain ground over the more secular Free Syrian Army. The oil rich Sunni powers of Qatar and Saudi Arabia generously funded and armed Islamists, whilst the FSA struggled for support. The rise of the Jihadis lead to other minority groups to side with Assad, fearing a future Syria under al-Qaeda.

These developments left the West with few options, if any. To arm the secular rebels, with the clear risk that the weapons could end up with Jihadis? To actively intervene, putting Western soldiers in the middle of yet another bloody conflict? To ignore atrocities committed by the Assad regime? If Assad falls, then what happens?

Faced with no good options, the West did nothing. Yet now things must change.

Following the chemical weapon attack by the Assad regime (yes, even Iran admits that it was Assad) it seemed clear that finally something would be done. Obama declared that his red line had been crossed, David Cameron announced Britain’s active support, and even France’s Francois Hollande agreed that the time had come for real action.

But why? Over the last week, again and again, people have asked the simple question of ‘ma nishtana?’ – What is different this time? Does it matter if civilians are killed by bomb, bullet, or gas? Surely a war crime is a war crime, and dead is dead? Why is this weapon different from all other weapons?

Newspapers and websites around the world have offered many answers to this question, but in my opinion the answer is simple. They are different simply because we say they are.

For the last century since the end of the First World War the international community has managed to hold on to very few rules. Genocides have been common, from the Ottoman massacre of the Armenians, through the Shoah, to Cambodia and Rwanda tens of millions have died. Yet since 90,000 soldiers were killed with them in the Great War, and especially since the first countries ratified the Geneva Protocols against their use, chemical weapons have hardly been used. On only three occasions, by the Italians in Abyssinia, by the Japanese in China, and by Saddam’s Iraq in Kurdistan has there been widespread use.

This record may not seem like much, but it’s value seems clear when you see it in historical context. Hitler’s Germany never used chemical weapons, neither did the Allies. Some say that this was because they don’t work well as weapons. But they did in WW1. Ninety thousand soldiers died and 1.2 million were injured by them on the battlefield – and that was before the invention of nerve gasses which would have dramatically increased those statistics.

Some of that conspicuous non-use can be put down to mutual deterrence, in the same way that MAD kept the peace during the Cold War. But that doesn’t explain why they aren’t used far more often in cases similar to Syria’s civil war.

The main reason that chemical weapons have not been used far more widely in the last century is because they are one of the last remaining international taboos. It isn’t done because it just isn’t done.

And there lies the reason why the world must act to punish Assad. Because if he can get away with it, clearly, and on a large scale, just once, he will do it again. And then he will do it again. And then next time a dictator needs to quickly put down a village, he will use them too. And then someone will use them in a war. And then it will be normal.

It is a far far better world when chemical weapons are not used. Until last week, that was the world we all lived in. If the world does not punish Assad quickly, then we will never live there again.