What's odd is that he wasn't alone. You will find similar sentiments all over Europe - take Leonid Andreyev's the Red Laugh (1904) or the Weltenende poem by Jakob van Hoddis (1911). Housman is not quite as outspoken but his poetry was gloomy as well.

What's behind all of this, historians?

Prior to WWI, war was still romanticized and seen as a sort of crucible that let men test their mettle and exhibit heroic traits like loyalty, courage, sacrifice, and so on. It wasn't a horror; it was an adventure.

There was a common sentiment in Europe at the time that society had become decadent and corrupt, and that peace prevented men from rising to the occasion.