There is a statue of Lord Castlereagh (1769–1822) in Westminster Abbey, but the inscription on his grave nearby has, with perfect melancholy symbolism, worn away. With the exception of the Irish, who forget nothing, Castlereagh is usually remembered, if at all, only for fighting a duel with a cabinet colleague and for committing suicide with a penknife, the sole British foreign secretary to have done so, by that or any other method.

Mainstream historians who prefer to run no risks might add that Castlereagh was a wicked reactionary, author of the policies that generated the insults that have kept his name alive in literature, if nowhere else. Byron labeled him an “intellectual eunuch”—a sneer made all the more cutting by his target’s childlessness—and rejoiced at his suicide: “Here lie the bones of Castlereagh/ Stop, traveler, and piss.”

Shelley,...