When war broke out in August 1914, crowds in Trafalgar Square cheered. In Germany, even the liberal novelist Thomas Mann exulted, “War! We felt a cleansing, a liberation.” The “world of peace” had bored him.

His words show how far we have come since. Most recent commentaries about 1914 emphasise current risks of war. Yet today's overriding reality is peace - more widespread internationally and domestically than probably ever before. Armed conflict and violent crime are declining, as the Harvard psychologist Steven Pinker describes in his seminal The Better Angels of Our Nature. What if trends are towards even more peace?

The notion that we live in peaceful times is counter-intuitive. Years after the attacks of 9/11, a woman from ostensibly calm Jupiter, Florida, asked me, “Don't you think this is the most dangerous time in history? My friends and I call each other, we hardly dare leave the house. We just pray our children are safe.” The mind springs easily to today's actual conflicts, because TV news is about bad things: terrorists, drones, Syria, incipient civil wars in South Sudan and the Central African Republic, murders everywhere.