"In some ways, you might be able to argue that the First World War started on the 22nd of April 1915.
"Up to that point, what had occurred was a series of well-known 19th Century encounters that had gone wrong. But on the 22nd of April, not far from the city of Ypres, the Germans did something new. They opened cylinders of poison gas to try to break through the defensive strength of the allies on the other side. French and Canadian troops were hit by this gas, or chlorine gas, and were terrified. These are men without really any protection against this because it never happened before. These weren't shells, these were cylinders that had been lined up, and when the German troops thought that the wind was blowing the right way, the cloud opened – it looked very much like a green cloud – and the people who didn't escape from it would have their lungs burned out and die an awful death.
"Now, that moment is very important in understanding how the war that so many people joined up to fight turned into something much worse.
"It's important for a whole series of reasons. One, is that gas warfare is another level of brutality, another level of violence that until that point, had not been available to either side. And once it became available to one side, it was used by both.
"But it also required a huge industrial backup to produce. By 1918, one in every four shells on the Western Front was a gas shell. That meant huge factories producing weapons of war of a kind, which lowered the obstacles to brutality, because if you couldn't get out of a trench, if you couldn't flee, if you couldn't surrender, then there is a different nature to battle and confrontation. That is extermination, not combat, because a soldier cannot surrender.
"If he has no protection against the gas, he will simply suffocate and die miserably.
"The nature of that kind of war opens up the whole issue of whether this conflict began in a 19th Century fashion with a degree of understanding about what the limits of violence were, and, slowly but surely – 1915 is a critical moment – those limits were pushed and pushed and pushed until they didn't exist anymore. So that by the end of 1915, you could say that everyone in each combatant country was at risk. No one was safe. Everyone was a target.
"The first of those steps was the bombardment of civilian populations through zeppelins. This is something that brought home to the civilian population that they, too, were on the firing line. A kindergarten in the east of London was not what the zeppelins aimed at, but given the state of the art at the time, it's not surprising that they killed children.
"It is in 1915, that this kind of war was born, and the best way to understand it and its horrifying character, is to call it total war."