Robert Frost on Why Nations Go to War

From a 1915 letter to English poet Edward Thomas explaining why Americans were (at the time) against fighting Germany:

But few consider the war any affair of ours. No one goes to war on general grounds of humanity. We extend sympathy on general grounds of humanity. We fight only when our material interests are touched. Yours were when Belgium was invaded; ours weren’t.

From the first volume of Frost’s letters, just released by Harvard.