That was the title to a Randolph Bourne essay found uncompleted after his death in 1918. It arose out of WWI, and Mendy McElroy penned a 1999 essay on the core meaning of the phrase. Essential reading; essential link in your collection.

The thrust of Bourne’s essays is to attack the sanctity of war by showing
how it leads to the moral collapse of society by kicking out the props
(the principles) of peaceful interaction upon which society rests.

In essence, Bourne addressed the moral consequences of war upon a post-war
society which had abandoned individualism in favor of "the herd-machinery."
He eloquently argued that post-war America would be morally, intellectually,
and psychologically impoverished. By this observation, Bourne did not mean
that peace time America would struggle under the increased bureaucracy
that never seems to roll-back to pre-war levels. Many historians have made
this point. Bourne addressed the less tangible, though arguably more significant,
costs of war. For example, post-1918 America would be burdened by intellectuals
who had "forgotten that the real enemy is War rather than imperial Germany."
In converting World War I into a holy war, the intellectual and psychological
groundwork was being laid for future instances of what he termed "the sport
of the upper class" — global conflict.

All of that, in order to get to my Christmas message for this year. After browsing around the net for something that would spark a bit of Christmas joy in me, I found I could do no better than this. It’s a tragic story, but that’s only because it’s chock full of humanity at its best. Soldiers Against War: The Story of the World War I Christmas Truce, by John V. Denson.

"Never . . . was I so keenly aware of the insanity of war."