IT HAS been at the core of the European project ever since its inception in the early 1950s. Even so the Franco-German relationship has worked largely by simultaneously disguising both German strength and French weakness. And it has had many ups and downs. Yet a combination of the euro crisis, German economic success under Chancellor Angela Merkel and French economic weakness under President François Hollande has made it more lopsided—and thus more fraught—than ever before.Consider the demonisation of Mrs Merkel across the euro zone. It is one thing for this to happen in the streets of Athens or Nicosia; quite another when it breaks out in Germany’s main partner. Yet Mr Hollande’s Socialist Party is lashing out at Germany, and specifically at Mrs Merkel.The latest row began with a draft party document, leaked to Le Monde, that used incendiary language in talking of a European project driven by Mrs Merkel’s “selfish intransigence”, called her the chancellor of austerity and said her policies were shaped exclusively by concerns for the savings of Germans, the trade balance and her electoral future. This followed a call by Claude Bartolone, the…