A book-length critique of the idea that the "long peace" means that the world in general has gotten more peaceful: Only the Dead: The Persistence of War in the Modern Age by Bear F. Braumoeller[0].
I came across the book recently, and it argues that we won't be able to statistically tell if the world is in fact getting more peaceful for about a century. The incidence of war as well as the number of dead doesn't seem to have appreciably decreased on average for the past two centuries, on a worldwide scale. It's true that there hasn't been a war as destructive as World War I and II since then (although the Iran-Iraq war rivals those in terms of number of deaths divided by the population of the combatant nations), but there's no particular reason to conclude that wars that kill so many people won't happen again.
I came across the book recently, and it argues that we won't be able to statistically tell if the world is in fact getting more peaceful for about a century. The incidence of war as well as the number of dead doesn't seem to have appreciably decreased on average for the past two centuries, on a worldwide scale. It's true that there hasn't been a war as destructive as World War I and II since then (although the Iran-Iraq war rivals those in terms of number of deaths divided by the population of the combatant nations), but there's no particular reason to conclude that wars that kill so many people won't happen again.
It was quite convincing and sobering to me.
[0]: https://www.amazon.com/Only-Dead-Persistence-War-Modern/dp/0...