Matteo Renzi, the prime minister of Italy who took the revolving presidency of the European Union this week, seems to be the sort of man that Napoleon was referring to when he reputedly said that the key qualification he sought in recruiting a general was good luck. Renzi become prime minister without even needing to win an election because Silvio Berlusconi and all other rivals self-destructed. He took power just after Italy passed the lowest ebb of its economic fortunes. In May, he was rewarded for his good fortune by Italy’s voters, who anointed him with a strong democratic mandate in the same European elections that discredited almost all Europe’s other national leaders. Now he is taking the helm in Europe, as an economic recovery is starting and the European Central Bank is swinging decisively in support of growth. But even a politician as lucky as Renzi could not have counted on his latest and most unexpected windfall: the unintended consequence of last week’s failed campaign by British Prime Minister David Cameron to stop the appointment of Jean-Claude Juncker as head of the European Commission.