Heart of Europe: A History of the Holy Roman Empire, the magnum opus of Oxford’s Peter H. Wilson, is almost a thousand pages explaining a thousand-year empire that few contemporary readers know or care about. Yet the long-ago, expired heart of Europe still has a vital message for the United States today. All any Brit or American can recall about this empire, which lasted longer than Rome’s, is perhaps Voltaire’s leitmotif that it was neither holy, Roman, nor an empire. Probably true—but that would be to list its strengths rather than weaknesses, notwithstanding the French cynic’s intention. It was not holy since…