To indulge, one more time, in the metaphor of war as a kind of living thing, a parasite on human societies: the idea of a war to end war is one of its oldest, and cruelest, tricks.

Having campaigned against the war in Iraq while emphasising the need to do more in Afghanistan, Mr Obama will face a continuing temptation to end the former war while reinforcing the latter.

As the war dragged on and Irish casualties mounted, however, their sympathy for the Union cause began to flag, and by the end of the war many had abandoned the Northern cause altogether.