European Commissioner for Internal Market, Thierry Breton: 'Europe stays open for business. But we’re being a bit less naive.' Photograph: Olivier Hoslet / EPA The idea that trade produces peace between nations has an old history in Europe. “Peace is the natural effect of trade,” the French Enlightenment thinker Montesquieu wrote in his 1748 work, The Spirit of Law. “Two nations who traffic with each other become reciprocally dependent.” In modern times this concept has been most closely associated with Germany, as a core principle of its foreign policy. The idea is known as “Wandel durch Handel”, or change through…