Among the millions of words which will be expended over the next four years on the first world war, very few will be devoted to explaining one of its greatest legacies of all, the effects of which continue to dominate our politics to this day. One of the best-kept secrets of the European Union is that the core idea which gave rise to it owed its genesis not to the second world war, as is generally supposed, but to the Great War a quarter of a century earlier.